r4223 in Private: Clean up properly when we get the SIP 486 response.

git-svn-id: http://wush.net/svn/range/software/public@4919 19bc5d8c-e614-43d4-8b26-e1612bc8e597
Roughly r4224 in private:
More reliable detection of busy condition on MTC.
Also adds new GSM state, "Busy Reject".

git-svn-id: http://wush.net/svn/range/software/public@4920 19bc5d8c-e614-43d4-8b26-e1612bc8e597