Please see attached guide for 3CX

Transfer Issues

Scenario: we received an external call on our 3CX, and do a "attended transfer" to an extension configured with Call2Teams. If we pickup the call with Teams, we have the internal person on the line, but when he does the "transfer" we stay in call, with with nobody and the external call stay in the waiting zone… 

Fix:  Go to the Options Tab in the 3CX admin panel and turn off Support "replaces" Header.