SIP call can be classified as follow:
In an Early Offer, the session initiator (calling device) sends its capabilities (for example, codecs supported) in the SDP contained in the initial Invite (thus allowing the called device to choose its preferred codec for the session).
For Early Offer, the answer is sent in a response (usually 200 OK).
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 in 200 OK message (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.
For Delayed Offer, the answer is typically sent in the ACK of 200 OK
CUCM can support Delayed Offer in one direction and Early Offer in the other direction. This capability can often be useful in situations where a SIP device connecting to CUCM via a SIP trunk wishes to control the codecs offered and selected for inbound and outbound calls.
What will happen if the calling device is using Delayed Offer and CUCM needs to forward the call over SIP trunk with Early Offer config? By default the call will fail because CUCM doesn't have media characteristics to relay them over SIP Trunk. Solution is to insert MTP where CUCM will use the characteristics of the MTP device over the Early Offer SIP trunk.