No RTP sending from IP Signaling on ProSBC

1. General
1

Hello everyone,

Our SIP trunk provider has informed us that the signaling IP of ProSBC, which is connecting to their SIP trunk router, is not sending any RTP.

We are currently facing an issue with one specific client who is peered through ProSBC. Here’s the scenario:

We have tested calls from three major GSM mobile providers to the client’s trunkline.

  • With the first provider, calls are intermittent—sometimes they go through, other times they drop, and most of the time we experience dead air.
  • With the second and third providers, we consistently get dead air.

This issue appears to be isolated to this one client.

Has anyone encountered a similar situation or have any insights that might help us troubleshoot this issue?

Thank you in advance for your assistance.

2

We are also experiencing this issue on a newly installed version 3.3.35

3

Yes, this scenario is quite common — especially if the client’s network involves NAT.

Another factor that may impact this behavior is the use of symmetric RTP, as defined in RFC 4961 and related to how some endpoints manage media path expectations.

Please note that ProSBC itself does not generate RTP — it simply relays the RTP it receives from one side to the other. The only exception would be if a transcoding unit is involved in specific scenarios. So, in your case, it’s likely that ProSBC is not receiving any RTP from the client side, and therefore it has nothing to forward to the SIP trunk provider.

To investigate this properly, I strongly recommend capturing the traffic using TBRouter, making sure to include RTP in the capture. If you can share the .pcap file via a download link along with the level 1 logs (level 1 debug report), we can take a closer look and help identify the root cause.

In most cases, the issue is related to NAT or ALG (Application Layer Gateway) interfering with media negotiation. However, sometimes it’s related to how the endpoint handles symmetric RTP, or to other SIP signaling behaviors.

Let us know if you can provide those captures — happy to help dig deeper.

4

You should try to upgrade to 3.3.6, or revert back to 3.1.147.