When calling outbound via SIP Trunk to ITSP you receive a 400 Service Unavailable-No Ports Available error after receiving the TRYING packet.
This error may indicate an issue with Early Offer being disabled. Most ITSP’s expect the SDP in the initial SIP INVITE. If the SDP is missing, the ITSP can return the above error.
Simple fix is to enable Early Offer on Cisco CUCM. Below is a quick how to.
- Navigate to the SIP Profile configuration page and select “Mandatory (insert mtp if needed) for the Early Offer Support field.
- Navigate to the SIP Trunk configuration page and select “MTP Required”.
- Reset SIP Trunk.
