Hello!

Check this – Flashphoner
It is the best Flash-SIP server.

It is only flash-to-SIP software which allow you make your own flashphones or click2call widgets in an hour.
Flashphoner support transcoding from Speex to G711, good sound quality and, certainly, support SIP 2.0

Flashphoner amazing simple integration, and has Free Trial Developer Version
Documentation here, forum here

Enjoy!