Timer C is only used for SIP proxies. It is used to make sure you receive a response for the proxied invite, if you dont receive a response before this timer has expired it will either generate a timeout or query alternate contact if multiple contacts exist. Default timeout is 3 min.
Refer to this IETF doc: https://tools.ietf.org/html/rfc3261#page-114
Timer C > 3min Section 16.6 proxy INVITE transaction
bullet 11 timeout
Also Refer here: http://www.sipknowledge.com/faq_child6.htm#q2.22
Since Timer C is started by Transaction Users, is the timer C also used in UAs and reset at the receipt of provisional response?
Timer C is a special one. It is only used by client side of proxies. Its purpose is to serve as a 'master timer' for stateful proxy, which may have forked, hence helping it to make dynamic decisions on the potential set of transactions created by it. The key timers which are correlated between client and server side of INVITE transaction are B and H (giving up on the transaction respectively). There is NO equivalent for timer C on the server side.
Cisco SIP Proxy:
http://www.cisco.com/c/en/us/td/docs/routers/asr1000/configuration/guide/sbcu/2_xe/sbcu_2_xe_book/sbc_str.pdf
Timer C > 3 min Proxy INVITE transaction timeout
-Terry
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