Confirm Trunk settings in your UCM are configured to the below values.
UniTel SIP Trunks should be created as type Register SIP Trunk using the Trunk credentials in the UniTel Dashboard > Settings > Trunks.
Trunk setting Auth Trunk must be unchecked.
Trunk setting DID Mode must be set to value "To-header".
At this time we do not yet have available step-by-step instructions for configuring UniTel SIP Trunks for the GrandStream UCMxxx IP PBX. After reviewing the information above, if you continue to experience trouble, please contact our support team for assistance.