Symptom
audio calls drop after 15 minutes (depending on the session refresh timer) if there is fax passthrough configured on the CUBE and a=silenceSupp:off - - - -" received in 200 OK.
This is because SME cluster does not have any media resources configured and there is a codec mismatch between the original SDP and the fax switchover SDP.
Conditions
Call flow: PSTN --> CUBE --> CUCM SME --> SIP trunk --> CUCM Leaf cluster
## Call negotiated with G.711ulaw initially. Passthrough G.711alaw configured under fax.
## ITSP sends 'a=silenceSupp:off - - - -' in the 200 OK for session refresh Re-Invite.
//5424/580DF173958E/SIP/Info/notify/8/sipSPICheckFaxUpspeed: Fax upspeed codec (g711alaw) negotiation successful for m-line 1
//5424/580DF173958E/SIP/Media/sipSPIDoPtimeNegotiation: Offered ptime:20, Negotiated ptime:20 Negotiated codec bytes: 160 for codec g711alaw
//5424/580DF173958E/SIP/Info/notify/8/sipSPISetFaxFlags: Fax passthrough negotiated
200 OK sent towards CUCM with G711alaw:
o=CiscoSystemsSIP-GW-UserAgent 4709 7507 IN IP4 x.x.x.x
s=SIP Call
c=IN IP4 x.x.x.x
t=0 0
m=audio 10026 RTP/AVP 8
c=IN IP4 x.x.x.x
a=rtpmap:8 PCMA/8000
a=ptime:20
a=silenceSupp:off - - - -
Workaround
> Change the audio codec preference list on CUCM to match what is configured on CUBE as fax passthrough
> Change the 'fax protocol pass-through' command on CUBE to match the original SDP negotiated.
++ Recommended workaround is to apply the SIP profile to remove "a=silenceSupp:off from the 200OK:
voice class sip-profiles 8015
response 200 sdp-header Audio-Attribute modify "a=silenceSupp:off - - - -" ""
Further Problem Description
