Transfer call, received on GXW4104

[logsim]

Using a GXW4104 and a BT line, 3CX system (37098.738), a Grandstream DP715 and a 3CX windows phone (12.0.36097)
Pick up the call on the DP715 - OK - have conversation
Transfer to the windows phone - connects and have attended conversation whilst music plays on the incoming call
Hang up the DP715 - windows phone says "put on hold" and all buttons greyed out
Incoming call - nothing but still connected

How can the windows phone pickup the call?

If the call is from a SIP line then it works perfectly

 

[eagle2]

Try altering 'Support Re-invite' and 'Support Replaces' into Advanced setting for your gateway.

 

[logsim]

Thank you Eagle2
Under profile1 I changed "Force INVITE" from no to yes
It now works