The entire issue is explained in complete detail in this white paper from Avaya.

https://www.sipcenter.com/sip.nsf/html/WEBB69TLRZ/$FILE/AVAYA-Converged_Communications_With_SIP_lb2343.pdf

Go to page 10 of the PDF file and what I am talking about is eplained very well. The entire document is a great resource for SIP technology.

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