way of handling DTMF in SIP. Totally undocumented, but implemented
in enough devices so we have to support it. 

Code by sergee, small changes by oej.

Closes issue #11049


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@89278 65c4cc65-6c06-0410-ace0-fbb531ad65f3