Additional references
The best reference for the SIP protocol is RFC 3261. To read the RFCs is a little bit boring and sleepy. (It is very good when you have insomnia.) You can find RFC at http://www.ietf.org/rfc/rfc3261.txt.
The OpenSIPS mailing lists can be found at http://www.opensips.org/Support/MailingLists.
There is a mailing list where you can post questions about SIP called SIP implementors at https://lists.cs.columbia.edu/mailman/listinfo/sip-implementors.
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