I have a problem with loss of audio briefly during a conversation

I have a customer that we have installed a freepbx system for. I am using Freepbx

The problem they are having is that during a call, they lose audio both directions for 5 to 10 seconds. This doesn’t happen on every call, just sometimes. When it does happen, it occurs after they have been talking for a few minutes. After 5 to 10 seconds of silence, the audio comes back. There is no noticeable call quality issues before or after the silence.

I’m leaning towards it being a network issue, as this is the only system out of 12 that we have installed over the last year that is having this issue. All systems are running the same version of Freepbx and use the same phones (mix of grandstream 2130, 2160, and 1400). I have no way to prove this to the IT person on site.

Now I realize this is not enough information for someone to help me diagnose the problem, but what I am hoping for is that someone may be able to guide me in the process of getting more helpful information (or if I’m extremely lucky, someone else has had this issue and can give me some settings to check or things to look more closely at). What are some things I can test for or some logs I can retrieve that may help with this issue? Would running a wireshark capture during a trouble phone call be able to provide me with some helpful information? What other information can I provide that would assist anyone trying to help me on this?

Thank you to anyone who takes the time to read this and can help me troubleshoot this issue for my customer.

P.S. I will be onsite at this customers facility Monday the 21st, so I will be able to pull logs and run network tests if it would be helpful.


I have seen that before, you’ll be happy to hear. The BAD news is that it was an issue with their router, it was doing strange stuff with uPNP and RTP port forwarding.

If this is an internal call issue - in that, the RTP isn’t travelling through any routers anywhere - then that’s really bad, and it means you have a switch somewhere that is broken.

Thank you for the quick reply, and I apologize, this is information I should have included in the original post, but yes, this is mostly internal calls.

Here is the full setup. It is in a school, that has two campuses. I have a system at each location, and the campuses are connected with a direct fiber link. According to their IT person, there is no router or firewall between the campuses, just a direct fiber link between switches with each campus on its own vlan.

The information I have been given from the secretary is that the problem occurs only on calls between the two campuses and the only time it has happened while I was actually onsite to witness it was a call between campuses as well.

If this is an RTP issue, would a wireshark capture log the loss so that I can present it to their IT person so he can check his switches out?

Ahha! I’m going to put my network wizard hat on here and say ‘Spanning Tree Recalculation’ now.

What (probably) is happening is that some switch somewhere is going ‘I think I have a better idea of what’s going on than you do, current spanning tree root bridge’ and sending out a bunch of STP packets and causing an election, which are causing interfaces to recalculate.

This is normal, and expected, and usually won’t cause problems, until you’re doing RTP across the link.

Basically, this is one of the edge cases where the default spanning tree config can sometimes cause problems. Normally the best way to fix this is to explicitly deem a reasonably core switch to be the root bridge master, and then put explicit costs on the links, rather than letting them be auto calculated.

tldr: Get your net admin to read this post :sunglasses:

Thank you so much for your help. I will be onsite tomorrow and their IT guy is supposed to be there as well, I’ll show him this response and see what he can do to help get this issue straightened out.

Thanks again, and I’ll reply to this thread if I have more questions about this issue.

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