SIP trunk Garbled Audio with MGCP

[ty.masse]

I have a sip trunk with a provider connected via a cube 2900 router. The router is connected to cucm via internal sip trunk.  When I make a call From analog Phones Connected to an Fxs port Customers are complaining that they Cannot hear me. the sound is really garbled.  I have tested that scenario by calling my desk IP Phone from An analog Phone Connected to an Fxs.  The issue only happens on Some calls. You may dial 5 times with no issue and you would get it on the 6th call. It's Very Random.  Again the audio is never garbled Incoming. It's perfectly Clear.  The the Network is not congested when this is happening.   It also doesn't happen on calls initiated From IP Phones.

Looking forward to your answers.

[Jonathan Schulenberg]

Run a packet capture on CUBE, assuming this is a flow-through scenario, and see if the stream looks intact - and plays - in Wireshark. If the problem isn’t visible on CUBE then it’s probably a network congestion (eg upstream internet saturation) issue.

[ty.masse]

Thanks for replying. I forgot to mention that we're not going over the internet. We have an MPLS connection with the provider.
Also I don't see any evidence that it's congestion related. Ip phones works just fine. Only analog devices have the issue.
I'm suspecting a codec negotiation issue betwwen the analog gateway connected via MGCP and the provider's SIP trunk but I'm not sure where to look.

[Jonathan Schulenberg]

A “debug ccsip messages” on CUBE will show you the dialogs including SDP negotiation.

[R0g22]

Take a pcap for the affected call and attach it here.