I ran into the same problem until I found the answer:
http://lists.digium.com/pipermail/asterisk-users/2004-March/040488.html
Either you have 'signaling forward unconditional' inside voice service
voip or in a dial-peer.
IPTel SEMS, Asterisk and many other SIP Implementations (including IP
Phones from Zyxel and Grandstream among others) don't understand SIP
Messages with MIME encapsulation. Cisco does this when it has too much
information to send (i.e., additional signaling info, specially if your
gateway is ISDN or SS7), and most SIP stacks don't implement that.
Regards,
E.