call disconnected with CUCM after a call transfer

[ikomous]

We have CUCM Cluster10.5.2 at the HQ and at our remote sites there are 78xx IP phones that register on them. When the CAC was configured on the CUCM for one of the remote sites it was noticed that a call  between two IP phones is in progress and we want to transfer to another IP Phones ; when the transfer key is pressed, the initial call is disconnected.
Thank you for helping me.

[tyroneclark871]

Sounds like an MTP issue - Check that you have a Software or Hardware MTP configured for the relevant MRGL associated to the respective device pools. 

The call is likely terminating because there is no resource to use for the call hold when transfer is pressed so the call is just cut off. You can also check in RTMT for Media Resource issues.