This was a bug affecting outbound SIP calls in Asterisk 1.4 and 1.6. This issue has been fixed in their latest builds.
https://issues.asterisk.org/view.php?id=13209
An upgrade should resolve your issues. Otherwise, you can specify dtmfmode=inband on your outbound trunks and force G.711u. However, this didn't work very well in my experience.