Anyone Using Comcast SIP Service?

Now that I’ve got my ATAs working with my Comcast PSTN lines, I’m thinking about contacting Comcast for pure SIP service. Anybody using them?

Comcast SIP Service Page


Comcast is a horrible company, I wish they would go out of business but theyve created too big of a monopoly, leaving us no other option but to use them for ISP. Customer service is the worst of any company Ive ever dealt with it, its maddening. SIP? They dont have a clue

I personally use BroadVoice and their customer service is top notch, and cheap - 5 lines $55/month. They answer the phone within minutes every time I call, speak english and actually know how to troubleshoot an issue without putting me on hold or transferring.

Quick story as an example of Comcast SIP knowledge:

I once had a router from them that enabled SIP ALG by default with no way to turn it off, and no documentation of SIP ALG existing. After hours of troubleshooting my freepbx and sip trunk with the trunk provider, I found that others had problems with the same router, so I replaced it with a new one.

I had more problems and eventually found out the modem also was screwing up SIP traffic. I tried to replace the modem but was unable to because Comcast requires they keep ownership of the modem so that they can do special configs on it for static IP business users. It took 9 hours consecutively of being put on hold, hung up on, and transferred in order to get someone who understood the problem (somewhat, at a low level such as "ive heard of people having this problem before and we fixed it by replacing with _____ modem model)

Finally the modem was replaced with an older crappy looking one and my SIP trunk began working

I swear they did all that just to drive anyone trying to set up a SIP service mad, so that they will pay the ridiculous fees of $50/line to Comcast for phone service

Also do some research on Comcasts reviews, they are by far the most hated company in their industry, possibly the world

Comcast receives more FCC complaints than all their competitors combined

A modem is a layer 2 device, it can’t “mess” with SIP traffic. If you had it in router mode and double NAT’d your traffic that would cause huge issues.

Supporting SIP traffic on their network and their SIP service are 2 totally different things.

I am by no way endorsing Comcast service just commenting that besides being a grammatical and punctuation disaster this message is also technically inaccurate.


I’m very well educated in SIP and networking with over 15 industry certifications including CCNP Voice + years of experience on VoIP projects ranging from 5 phones to 125,000 phones

I am replying from my mobile device so sorry if I offended anyone with my grammar, actually I dont care

I know the difference between all 7 layers of the OSI model and know double NAT vs public IP, and I am telling you this modem interfered with VoIP, i will give supporting evidence later if I get bored and @skykingoh wants to argue it

I can assure you that Comcast is a shit company with plenty of supporting evidence aside from personal experience, but again Im on a mobile device and dont care go thru and cite my sources, just trying to give some support to the community before I crash out for the night

I would recommend any service other than Comcast… But thats just my opinion

I’m very well educated in SIP and networking with over 15 industry certifications including CCNP Voice + years of experience on VoIP projects ranging from 5 phones to 125,000 phones

Should I be impressed?

I would love to see your “supporting evidence” that a modem is modifying layer 3 packets. I bet Comcast isn’t going to let you put a sniffer on the CMTS.

on the other hand is off topic in this thread and contributed nothing of value to the OP

So is your comment, the OP asked about Comcast SIP service not their Internet service. For all you know they could have Business Class Fiber.

I am replying from my mobile device so sorry if I offended anyone with my grammar

My mobile device has punctuation and a shift key. Maybe you should look into that first.

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I am telling you this modem interfered with VoIP

A modem is not a router. Routers often contain modems, so this may be where you are confused.

@xrobau @SkykingOH

Okay smart guys, not sure why you have to insist that I dont know the difference between a router and a modem.

Here is what happened:

The ROUTER above (Cisco DP 3939B) was in use at a client’s site with Comcast/Xfinity business internet. I was installing a SIP Trunk, but having problems with it. I searched through the router’s documentation here, looking to see if SIP ALG was enabled and how to disable it:

The documentation mentions NOTHING about SIP ALG,so I assumed that it does not have it enabled, continue troubleshooting for many hours with the SIP Trunk provider. I then decide to use my own router just in case, but learned from Comcast that I have to use their modem.

So I put the Cisco DPC3939B into “BRIDGED MODE” which, to me, and any other network engineer would mean LAYER 2 MODEM. Apparently Comcast thinks so too, but they are wrong also, see their tutorial here:

“When you bridge a gateway, you are basically removing the extra features of the gateway and turning the device into a plain modem.”

This is actually a lie which I will explain below.

I assigned the public IP to my router’s WAN interface, so it did not appear to be double natted in any way and I had no reason to believe it would be, but continued to have SIP problems until the modem was replaced by Comcast with an older one after 9 hours straight of tech support with people who didnt even know what SIP was.

However, Comcast/Cisco made this very special router/modem with LAYER 3, SIP INTERFERING BRIDGE MODE.

According to this reddit post, Comcast has to use LAYER 3 BRIDGES as modems for their static IP business clients, it just happens that their Cisco bridge has SIP ALG enabled by default with no documentation saying so, and no way to disable it.

From Reddit:

With Comcast, at least in our area, the modem has to be a router for the statics to work.
Say they give you The modem will get it’s outside (internet facing) IP via DHCP and will have set as it’s inside IP. You can then set with a gateway of on your equipment.
By default there is also a subnet on the inside that is natted to When they put the modem in “bridge mode” all they do is disable that subnet.

I’m not going to cite too many sources for Comcast being the worst ISP customer service because all you have to do is google it to find the facts, but here is where I said they have more FCC complaints than all of their competitors combined

Comcast received 11,812 complaints so far this year (from January 1 to November 9). AT&T got 3,896 such complaints, Verizon got 1,588, and TWC had 1,240. In total, AT&T, Verizon, and TWC received 6,724 such complaints—5,000 fewer than Comcast alone.

I changed my mind, you should probably try Comcast SIP service because it is unlikely you will be able to get any other SIP service working. Just know youre dealing with a bunch of idiots over there, and if your service goes down you could spend 9 hours on the phone trying to get ahold of someone who knows something.

If you choose to try to setup with a different SIP provider, just keep in mind all the tricks above that are probably intentionally there to make it difficult for you to use SIP service other than Comcast.

See also this post where the customer found that Comcast was blocking ports 5060-5090 so he had to use a higher port to get around them

No, comcast does this from time to time. This usually happens around the time they roll out Comcast Digital Voice in a particular area. They’ll block vonage and make it appear that the Vonage service is problematic when in fact they’re just blocking ports. Then they call their customers in the affected area to upsell the digital voice. After numberous traceroutes and examples are provided and sent to Comcast NOC the “routing issue” is corrected and everything works again. Sometimes they go so far as to block out our website. It may not have been a large area affected so it may not have made the forums but this poster is most like telling the truth.


Comcast/Cisco apparently are the ones confused about what a MODEM vs ROUTER is, not me, they are the ones that made a router that secretly blocks SIP, and appears to act like a modem but isnt one. AND THEY EVEN CALLED IT A MODEM – look at the docs it says its a modem!

I never said a modem was layer 3 or was capable of interfering with SIP traffic, I just said THEIR modem was interfering with my SIP traffic, and I was right, their “modem” runs SIP ALG with no way to turn it off

More references:

Here is the follow-up on this. The DPC3939B is just a dud when it comes to VoIP. We were having VPN problems, too. We replaced the device with an Arris SB6183 (16x4) modem and TP-Link Archer C7 wireless router. These are working great and our VoIP and VPN problems are gone. Thanks for the advice given here.

We replaced our DPC3939B with an Arris SB6183 and TP-Link wireless router. The VoIP is working great and another problem we were having–disrupted VPN sessions–is also solved.

Well, quite a torrent in response to my question. :slight_smile:

My service happens to be Comcast Business internet (I needed a fixed IP, only way to get it) plus Comcast Business phone. So that should help. Their business side is a whole different thing than their comsumer side. And yes, they charge more for everything. First steps have not been encouraging. I filled out the request for more info form. No one called back. I called the number: “Uh… let me find your rep in the area.” Great Expectations… not.

I finally got through to someone at Comcast regarding their SIP service. They cast this as an enterprise service. The minimum is 6 channels at $35 a piece. So you can’t get started with them for less than $200 a month.

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I’m the one you are referencing in those posts and yes, the DPC3939B all-in-one modem / router / ATA / wireless is bad news for VoIP–that is to say, any VoIP except Comcast’s offering. Unfortunately it is the standard issue around this area for Comcast Business Internet subscribers.

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Lol thats outrageous. Go with BroadVoice – 5 Lines / $55 month (TOTAL not each!) and they answer the phone every time I call within 2 minutes, speak english, understand VoIP, and get the issue resolved without having to forward to “Level 2” support.

@billsimon nice, yeah I was dealing with this issue in October 2014, and when I googled it back then there wasn’t anything coming up at all about people having problems so I thought I was crazy until the modem was switched and SIP started working with the new modem. Now I see a few reports out there like yours, but surprised there is not more complaints.

You gotta love that Comcast rep at


“Comcast DPC3939B does not block any SIP Ports especially ALG 5060”

Ah yes the ALG 5060 port is not blocked. Good to know!


" If you are having an issue with one of your SIP devices where you believe the embedded SIP subpacket within the Ethernet Packet is being blocked, I would first use your SIP analyzer to make sure that SIP ALG is disabled on the SIP device."

"Perhaps you need to examine the SIP subpackets a little closer to insure that the presribed implementation ports a being used AND are all correctly opened on the VOIP devices and in the DPC3939B IP address devices. "

This should be evidence enough for why you should never use Comcast SIP service.

Support loves to babble on and on, and throw in as much meaningless technical jargon as possible, with absolutely zero understanding of anything theyre actually talking about

the easy fix for Comcast is to put in your own modem - i have not run into issues of Comcast blocking sip traffic, but the cisco modem they use really sucks. the arris (Motorola) surfboard modem runs great and is moderately priced. the install is easy as well. plug it in and register it

In fairness, that’s not a Comcast employee. It’s a forum babbler. There’s one on every forum. :slight_smile:

They require you to use their rented equipment if you want static IP service. If the only equipment they are offering breaks SIP like the DPC3939B then you are out of luck.

That’s what I was going to ask. When we installed our static IP I asked about using our own equipment and they said no. That seemed wrong, I thought they were required to allow user owned equipment, but I wanted confirmation here.

Ah ok I will give them that, but in all honesty, that forum babbler probably knows more about SIP than an actual Comcast Support rep

@bksales @kbocek Yes would love if we could use our own modems like Surfboard which is great, I use it on my residential connection but as others said Comcast forces us to use their modem if you have a business connection + static IP block

It was very difficult to get a non Cisco DP modem because they are the new ones that Comcast uses everywhere, but I got lucky and the tech that came out had an old SMC modem in his truck

yes the old smc modems worked pretty well. i could have sworn that we put in our own modems on comcast business with static ip’s but i will go back and verify.

I can confirm the same nonsense with Time Warner. Time Warner support was aware of the SIP ALG features and disabled it in their device (an Arris) and things worked better.

Unfortunately, the TW network around here (Milwaukee area) isn’t great. I do have two Comcast clients (Chicago and Atlanta) and their networks have been terrific.

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A third party SIP provider can provide lower rates but it is a best effort service. The owner of the network can provide end to end QoS. To them SIP is just a different delivery n method of tariff level voice service so they don’t discount it. It’s just a transport line PRI to a telco.

Integrators love SIP because you don’t need specialized interfaced cards.

Comparing and carrier to an ITSP is not fair.

Somewhere in the middle is what I do, we peer with the carriers. I offer on net service for Time Warner Customers (some west coast markets are not I n the t-bone and are excluded), Level 3 and AT&T fI ber and MIS customers. With peering we too can offer meaningful service level agreements.

Universally there is a little pain getting SIP trunks up with carriers like Comcast because their provisioning folks are not up to speed, but once up them are rock solid. I would be loathe to blame the carriers CPE equipment.