Phone calls fading in and out and dropped calls

We are having a problem where the incoming calls are fading in and out and occasionally gets dropped. We have the provider running traces on their network traffic (dedicated SIP line to PBX) but i am wanting to run some traces on the PBX end but i am unsure how to do this correctly.

We have 8 phones, FreePBX Distro V13, and a dedicated SIP connection to a local provider.


please explain what you mean by a dedicated sip connection? does the ISP have a router inhouse and you plug directly into their router or is the connection going over the internet?

@bksales Yes that is correct, the ISP brought in a dedicated router and modem for our SIP connection. They have been running call traces and they describe some RTP packet anomalies that they claim are coming from the PBX so I am going to run a tcpdump today to see if i can capture anything. Is this the best course of action to trace this or should i be looking elsewhere?

is this truly a dedicated connection? often times i see an isp deliver a connection that actually serves both voice and data. there is a data connection on the router and a voice/sip connection on the router. i chased a similar problem a while ago. it turns up it was mostly the ISP - they were not properly managing the bandwidth across their circuit. it is unlikely but possible that you have an issue in your internal network that is creating a delay. you probably want to check your trunk definitions and your network definitions on the pbx. i assume that you are using the 2nd NIC for the connection to the ISP voice port and the primary NIC for your pbx internet and phone connections?

Yes that is correct on the Network config. My understanding from the provider is the modem and router are on a range that is dedicated on their coax system to strictly SIP (althought i am sure they stack other data on it). They have already come in and run a new service line from the street to the building because of RF issues but we are still experiencing calls fading in and out. The strange thing is that it only seems to happen on inbound calls. Outbound are working just fine.

I am double checking the allowed codecs for the phones but I am unsure what else to check on the SIP Trunk definitions (the provider isnt helping with settings) or the network definitions. I can send over the current setting if you want.

coax? who is the provider?

and none of the providers will help with the asterisk trunk definitions.

The provider is Shaw Cable.

i am not familiar with Shaw so i can’t comment on their implementation. when you say calls fade out, i assume you mean that part of the conversation gets lost? when you say inbound calls experience the problem and outbound does not, do mean that if you dial from the phone system to an endpoint outside the office that everything always works perfectly but when someone calls into the office from outside that you have problems? or do you mean that only people inside the office hear the call fade out?

I’m concerned about the term “fade out”. I really think you need to tell us what you think that means.

Digital calls can’t “fade out”, at least not in the classic use of the term. They can break up, they can fall apart, they can stop having audio, but “fade out” is an analog problem. If you are hearing this on calls, it is either happening in your instruments (as in they are physically failing) or it is happening much further out than your system.

Unfortunately I am relaying the feedback from the client so I can’t comment on what the audio specifically sounds like. However I did receive an update from Shaw who was running traces on the calls and they found RF level issues at the modem which apparently can cause audio stream packet loss so they are attributing the calls audio fading or loss to that. There was never a call dropped during the “fading” issue just audio problems.

The dropped call apparently was due to a comm loss at the other end. The keep alive was not responded to so the provider hung up the call.

Sorry for any confusion here, the provider had stated that their trace indicated that the packet anomalies were showing coming from our PBX so I wanted to run packet traces to show our end hence this post. I appreciate the help but it looks like we have resolved the issue to this point.