I'm having some trouble with announced call transfers.

It works if we make local calls on our system and it also works with calls that come in through the PSTN line but if any calls come in from the SIP trunk it fails when trying to connect the lines together.

We are using the method that draytek have suggested for attended calls:

A calls B
B puts A on hold
B calls C
C is ok to talk to A
B presses transfer button

I've tried a couple of variants of this method but as far as I can see the issue seems to be with the SIP line.

We are using an IPPBX 2820 and Snom 320s.

If anyone could shed some light on this it would be appreciated.