I am facing the issue and do not know how to fix.
External phone call -> ISP SIPT->CUBE->CUCM->IP Phone A
When IP Phone A click transfer button to IP Phone B, external phone be hold. When IP Phone A click transfer button again to transfer the call, external phone is disconnected.
I have no idea about the issue.
Could anyone have some idea and help me to fix it?
It could likely be an issue with MTP or transcoder allocation failure during the transfer. Please check if Media termination Point Required is checked on SIP trunk and if correct MTP / transcoding resources are present in the MRGL. If you make any changes then reset the SIP trunk and test again. If the issue persists then post the detailed callmanager service traces so that the SIP signaling and media resource allocation can be checked.
Well noted your point.
On the SIP trunk, I had already add the media resource on MTP.
But I am no sure whether I need to check the "Media Termination Point Required" checkbox in CUCM SIP Trunk configuration or not.
You can go to the SIP profile applied on the SIP trunk and set the "Early Offer support for voice and video calls" parameter to Mandatory (insert MTP if needed). With this setting Unified CM inserts a media termination point (MTP) if the media characteristics of the calling device cannot be determined (for example, for an inbound Delayed Offer call forwarded over an Early Offer SIP trunk).
Collect below debug
++ debug ccsip messages
++ debug voice ccapi inout
From CUCM RTMT, Collect Cisco Callmanager Logs
Seems media resource unavailability issue...try to check MTP on sip trunk (CUCM-CUBE) and make sure mrgl with mtp resources is assigned to sip trunk as well.
In CUBE try this command
voice service voip
no suppl sip refer
no suppl sip move
MTP is usually needed for H323 transfers but not for SIP. Try to fix it without MTP (unless you are using different codecs between call legs)