Did you try searching google for sip traffic generator
The first hit is : http://sipp.sourceforge.net/
SIPp is a free Open Source test tool / traffic generator for the SIP protocol.
SIPp can also send media (RTP) traffic through RTP echo and RTP / pcap replay. Media can be audio or audio+video.
SIPp can be used to test many real SIP equipements like SIP proxies, B2BUAs, SIP media servers, SIP/x gateways, SIP PBX, ... It is also very useful to emulate thousands of user agents calling your SIP system.
I don't understand why people don't do a simple search before asking in a forum.