Pointers Transitioning From POTS To VOIP


I am wondering if there are any how-to articles out there to help me understand the transitioning from physical phone lines to virtual sip trunk type VOIP. I am at the point that I am trying to understand the concept of available phone numbers to concurrent connections.

If we have 8 physical lines and wanted to keep the same availability, I assume we would get 8 concurrent channels. I think that would mean we could have 8 in and/or out conversations at one time. If we now have 3 physical lines dedicated to customers and two for vendors, what happens to a potential phone customer if eight vendors call in at the same time?

That is what I am wondering about and didn’t know if an article or two are out there that would help explain this and other aspects to me.

Any help appreciated. Thank you.


There is no connection between these two things unless you choose to limit yourself in this way with a crappy SIP trunk provider that only sells service on a per trunk basis.

Buying metered service will be cheaper 100% of the time for the normal small business.

You pay $X per DID and $0.00X per minute and have no limit to the number of concurrent calls.

In your scenario, you likely do not need 8 DID. Just one, maybe two (fax).
At full retail, someplace like VoIP.ms comes out at $100/month if you use 10,000 minutes in a month.

There are certainly even lower costs available through services that do not have public pricing.


Thank you very much! When I see pricing per user from other providers, I am assuming my IP PBX is the one user (to the provider) no matter how many extensions I have connected to the IP PBX. Is this assumption correct?


No. It means per user. Those are horrible solutions if all you want is a phone system. Never pay per seat unless you really know the math works for you, and it rarely does.

If you are after a full UCaaS (Unified Communications as a Service) solution then those prices fall more in line.

Buying Ring Central or 8x8 at $25 per user is a great deal if you are using all of the features included collaboration and chat and voice, etc.

But if you are only buying voice service that is the wrong thing to be looking at.

For voice only service you have your own PBX some place. Since you are here, one assumes FreePBX.

With voice only services, you have two choices for SIP trunk providers.

  1. Pay by trunk with each trunk limited to a single call
    a. This usually costs ~$20+ per trunk
    b. This is typically an “unlimited” trunk, but there really are limits as there is no such thing as free service.
    c. This billing scheme matches the legacy “per line” billing of POTS.
  2. Pay by minute.
    a. This is the most cost effective way to obtain voice service for well over 90% of businesses out there today. Just do the math.

Option 1 is simply carriers padding their pockets because:

  1. There are no hard limits to the number of simultaneous calls a single SIP trunk can handle.
  2. There are no hard limits to the number of DID a single SIP trunk can handle.

I wish I did more homework before initially signing up for managed cloud-based VoIP service. Paid per-user and even per-feature if we wanted anything other than the stripped-down base feature set. It was a rushed decision since our on-prem PBX was steadily failing more and more frequently. When the contract term was up I definitely didn’t renew and looked for just basic SIP trunking. Unfortunately I am paying per-channel, but will look to connect with a provider for can do per-minute. Even then I am saving our companies ~$35K a year even with per-channel SIP trunking at the moment compared to the total managed cloud-based solution we had previously.

Excellent information! Thanks again. Yes, I have a FreePBX server running at home while I work on this solution for my place of employment. Basic phone/voice service is all we need. If I can add voicemail w/email, IVR, specific routing, etc., that’s all more than we have now. I see that the basic SIP trunking is the way to go paying by the minute. Since we have flat rate now, I think the ones to approve a new system would be turned off of pay by the minute. I will pull our records and see how many minutes we do use to better support a solution.

And, our phone system is dying slowly. I have replaced the system board in the PBX twice now (with used units) and the extensions are beginning to fail and am having a hard time finding replacements that work with our current hardware.

I appreciate the knowledge you guys have provided!

Only because they don’t math. People are stupid about this.
I posted about it years ago on another community

I have spoken about it more than once at conventions. Some place near the 30 minute mark in this video.

Until you display “apples to apples”. Take the number of minutes you use the phones in the office (if you can’t get a detailed bill, assume 640 minutes a day) and then figure out the “per minute” cost of those flat rate lines. Then, take your “per minute” cost and multiply it by the number same number of minutes per month.

This way, you get the “per minute” charge for both systems and the “monthly” for both systems. If my experience is any indicator, I think you’ll find than you are paying most of a cent per minute per phone whether they are in use or not and your SIP rate should be in the tenths of a cent for your SIP lines.

Most of my customers end up with phone bills for their entire enterprise in the $30-40 a month range (for about 50 phones). The same in a flat rate “10:1” configuration would be $250 or more per month before you add anything for long distance and 800 services.

This so totally makes sense. I just looked at my SIP trunking bill from last month. 25 channels at $20 per channel comes out to $450 for last month. Last month’s inbound/outbound call consumption amounted to 14,350 minutes. If I had a SIP trunking provider that offered, say, $0.01 per minute then I’d have only paid $144.50 for those calls.

That’s about 3x the normal rate.

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Sure, there are cheap services out there and they are not too expensive, either.

Incoming, maybe. AnveoDirect is $0.004 and very reliable, but you’ll have to get your 911 elsewhere. SignalWire is $0.00325 but they don’t have good failover options (for when your PBX is down or unreachable).

Outgoing, I’ve tried several in the $0.0025 range and all mishandle at least 1% of calls. You can get down to ~0.2% at ~$0.004, but you’ll waste time tweaking settings as new issues arise.

If you are B2C so nearly all your outgoing calls are to mobiles, you can get quality termination for ~$0.0025/min.

You get what you pay for.

There are solid, reliable carriers that offer much better than penny a minute rates.
But penny a minute is a good “retail” rate that anyone can buy off the street without digging.

All good to know. It’s all a series of singles, doubles, triples, and home runs regardless compared to our previous cloud-hosted solution. We had to connect call paths through the provider’s branded MPLS network, which consisted of bonded T1’s (no fiber) and added a few hundred dollars a month per span. Then we leased the IP phones, paid per-user subscription fees, etc. Adding all of that up, each of our larger sites was paying around $1,000 a month. Of course it was a “turn key” solution where it was all managed and warrantied. But exploring the alternatives quickly made it apparent that we were being gouged.

When you calculated those savings was that a straight up comparison of your UCaaS MRC vs your SIP Trunking MRC? Because if that is the case, that is skewed. You now have a software PBX that requires a server, power, space, etc. etc. all of those things are a factor. In some cases they could be a bigger factor than others. You also have the cost of being the admin of the PBX. So anything that your provider may have done at your request, you now have to do. So your time is now a factor in this calculation. Updates, changes, support for issues that happen.

Now that said, UCaaS could cost more than having a onprem PBX system but there are various factors that need to be considered. I offer both services in the markets I’m in and they both have their advantages and disadvantages. In some cases UCaaS works out better for them cost wise than their onprem PBX. However I should clarify I’m in markets where telecom level support is expected and desired (24/7 support, onsite calls/truck rolls, etc). So 95% of the onprem PBX systems are hardware based for those I supply SIP Trunks to whether is has a PRI or SIP interface on it.

Something else to consider with onprem, from my perspective, I have less than a dozen places where I am End-to-End (Voice/onprem PBX). Roughly a 1/4 of them are FreePBX and the rest are hardware based systems. There is more cost to the maintenance of those FreePBX systems than there is for those hardware systems. By that I mean I don’t have to touch those other systems at all once they are up unless it’s a programming change (which is rare). On the flip side, I’m in those FreePBX boxes at least once a month doing updates and maintaining them.

Again, MRC to MRC from the carrier there is a difference but you always have to calculate everything else that you take on when doing onprem systems.

And I haven’t even touched on the costs of backups or any DR plans/policies.


Very true. The FreePBX itself is a VM that runs on an existing blade, so it shares environmental resources of the chassis as a whole. VMware local and cloud copy backups include the VM, which isn’t in and of itself very large. My own time administering FreePBX is likely the largest factor to take into consideration. I’d guestimate that occupies roughly 5-10% of my overall workload. I’m a salaried drone, so as long as I balance all of the tasks that occupy my time it’s good so far. Just like back in the day when we had Merlin Legend and Magix PBX’s deployed at all of our sites :smiley:

The providers I use (VOIP Innovations, Alcatel Networks, and Telnyx) all offer considerable savings over $0.01 per minute providers and offer excellent customer service. VI (and I think the other two) offer 911 services at a monthly per-number rate. You could do a lot worse than any of them.

Agree 100% with prior remarks - metered, unlimited channels is almost always less expensive than per channel or per user.

Do all outbound calls present the same number, or if you dial out on “Line 8” does it show a different number than “Line 1,2,3,etc?”

If they present a different number, you may want to port and keep all 8 existing DIDs to start - a lot of folks will have those non-primary numbers saved in their contacts.

We generally set up the non-primary numbers with a “number has changed message” that automatically rings through for the first few months, then a “number changed - please hang up and dial” message for a few additional months, and eventually retire based on the the CDR activity.

DIDs are cheap though, and some folks will keep the old numbers around indefinitely vs inconveniencing a customer or risk loosing a call.

Unfortunately they just changed their policies on that. Not sure they are trying to be a place for little guys anymore. They want $100 MRC commitment now.

Yes - I better understand the different options and pricing now.

Our outbound indicate the POTS line number selected. Line one is advertised mostly and will roll over to lines two and possible line three if unavailable. I like you idea to at least reduce the number of DIDs in time. I think it would be nice to have one intended for customers and one for vendors to better handle the traffic priority - I could program option accordingly.

We also have a separate fax line: Can we use our current POT line fax machine with VoIP using an ATA device to connect to the PBX if using a non-compressed codec? I see reference to T.38 compatibility. We get enough faxes that this would be important. I assume a HylaFAX server would not help any compatibility issues.

If you have a pots fax, leave it just the way it is, connected to your fax machine.