[SIPForum-discussion] Fax call getting 488 not acceptable here

Murali Natesh murali87ece at gmail.com
Wed Oct 23 11:38:07 UTC 2013

Hi guys,

I have one fax call scenario mentioned below

PSTN-----*SS7*------>SOFTSWITCH-------*SIP*----->SIP Trunk with customer

PSTN user sending the fax.The audio has been established using G.711 Alaw.

 while the called SIP user sending Re-invite with T.38 in SDP the calling
party  giving 488 not acceptable here the ACK is given by Called SIP UA.

It means the Calling UA not support T.38 may be the PSTN.There is no INVITE
from Called SIP UA to continue the Session with the existing G.711 Alaw.

Please clarify me the following things.

1.Do we have to ask the Called SIP carrier to enable the fallback procedure
from T.38 to G.711Alaw.do they need to send INVITE with G.711Alaw after 488
not acceptable here?

2.Do we need to add any codecs in the PSTN trunk to negotiate with T.38?

Thanks and Regards
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