One way audio when not picking up transferred call

we’re experiencing a weird behavior with our asterisk installation, we’ve tried everything but we can’ figure out the problem.
This is the scenario:
A call from an external number has to be transferred to another extension using attended transfer.
So the first extension makes a new call to the second extension and ask for the permission to transfer the call, and then hangs up so the external call is transferred to the second extension. The problem is when the second extension doesn’t want to receive the external call (or doesn’t answer) and the first one has to pick up the external call again (which was on hold). In this specific case the call becomes one way, where the external caller can hear the internal, but not the opposite.
Everything else works perfectly, exactly as expected; event the same situation, if the call is made on the opposite direction (from an internal to an external number) works fine.

Any idea on what may be wrong?
Thanks to everyone in advance, if you need any further detail ask.

Technology? Method of requesting the transfer, if the technology is SIP?