Alan's sysadmin Blog

Working smarter not harder

PRACK causes No Ringback with Lync

Posted by Alan McBurney on September 12, 2012


I ran across an issue with a Lync Enterprise Voice deployment whereby no Ringback tone was being heard by the caller although the call would complete and could be answered.

This issue only occurred when calling from PSTN\Mobile across SIP to Lync.

An SBC was in play to convert from SIP UDP to TCP before sending to Lync. The SBC in question was a Disalogic BordNet 500.

After a lot of packet capture and analysis the issue turned out to be the SBC’s B2BUA being configured to Receive PRACK.

With PRACK enabled no Ringback was heard, after disabling the setting Ringback was heard by the caller.

 

 

 

 

 

 

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