Some questions:
1) Does this work for any extension?
2) If it does work does it work only for certain phone models?
A couple of things: (most of which are just guesses on my part)
1) be sure the extension you are dialing out from is in the DID range.
2) In checking the pcap I can see in the invite a "Require: 100rel" - pure guess on my part but in the SIP profile turn off Reliable Provisioning.
Ralph