[cisco-voip] Cisco SIP phones

[Aman Chugh]

We have been working on a strange Issue with TAC , We have CUCM
6.0.1.1000.37 , The Problem which we are facing is that when we make a PSTN
call from a SIP phone 3951 to a cell phone or PSTN Phone, Ring back tone is
played on 3951 IP phone even if the mobile phone at other end is switched
OFF & PSTN  is busy, A 7970 sip phone does not behave this way. I have
sniffer capture for this and I am not good at understanding  SIP messages.
TAC engineer has told me to enable PRACK message , can any one tell me what
does this SIP message does and how to enable it on CUCM.

TIA

Aman
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