From my understanding UCM can only due Delayed Offer or an Early Offer with an MTP. Delayed offer does not send an invite with SDP, which is why it's referred to a Delayed offer.
This is from the UCM 7.1 SRND;
In a Delayed Offer, the session initiator does not send its capabilities in the initial Invite but waits for the called device to send its capabilities first (for example, the list of codecs supported by the called device, thus allowing the calling device to choose the codec to be used for the session).
Media Termination Points (MTPs) are generally not required for Delayed Offer calls from Unified CM SIP trunks. For this reason, Cisco recommends Delayed Offer as the call setup method for outbound calls from Unified CM SIP trunks. For outbound Early Offer calls from Unified CM, MTP resources are required (SIP Trunk
MTP required box is checked and the desired codec to be offered is selected) and remain in the media path for the duration of the call.
Hope this helps.
We use the Cisco CUBE for our SIP Trunks to SIP Providers so that we have better control of Codecs, Call Flow, SIP Messages, Etc...
Mike Shelley