Does anyone know if its possible to forward calls to an external number using this feature of the Phone?

Ive upgraded to the latest firmware and tried forwarding to a mobile and a land line but when I call the forwarding phone, the line just disconnects and goes dead.

We are using the ippbx2820 with ISDN2

I've seen it not work with 3.5.6, try downgrading to 3.5.5.3 using firmware from ftp.draytek.com and see if that works?