No Audio or One-audio - SIP Incoming Call Forwarding to External Number via SIP trunk


  • Firmware version: or above
  • Model: S-Series PBX, K2, P-Series
  • Call incoming then forward via  SIP trunk to an external number.

Problem Description

Everything works like a charm in S20 before upgrading to from

But voice issue occurs after the upgrade when calling from an external number to the mobile number of the mobility extension.

Call flow example:

Calling party A ----- Inbound -----> PBX Forward the call ---- Outbound ----> Called party B.



When it comes to the PCAP log, an RTP(CN) is sent in the beginning by S20 to get through the communication channel of the call. This is for "pin-hole" and activates the temporary NAT session of the router so that the RTP from the provider can be sent to S20. Therefore S20 and port forwarding of the router is not required.

Nevertheless, this RTP(CN) mechanism is not compatible with all SIP providers, which had caused voice issues between S-Series and some SIP providers before. In the newest version, the RTP(CN) will be sent only when LCS (Linkus Cloud Service) is used to avoid compatibility issues. Without the RTP(CN), it is not able to activate the NAT session which leads to the no voice issue in this case.




Choose either one according to your environment:

1.  Enable the  RTP Keep-alive option for the SIP trunk used to forward the call.


Note that the PBX firmware should be X.14.0.127.

For P-series, Check the similar option.


2. If you don't want to upgrade the PBX firmware. Try this solution instead.

Enable the RTP Keepalive option to overcome the "pinhole" difficulty.

1) Create  pjsip_custom.conf in the /ysdisk/support/customcfg/ via SSH


2) Fill in the content in pjsip_custom.conf


Save the file.

3) Input 2 commands to let the change take effect.

asterisk -rx "module reload"

Then test again.

Or you could refer to this video to learn how to enable it: Customize RTP Keepalive Option in PBX


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  • 0

    I had the exact problem described in this article on a S20 with firmware Enabling the "Inband Progress" on the SIP trunk solved the problem right away.

  • 0

    Dear Shay,

    thank you very much for this article, we spent many hours to find this issue, now after enabling the inband progress everything is back on track.

    How could it be that a (automatic) firmware update is able to make such a problem?!    

    Edited by mj
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