Problem with transfering a call to sip trunk

hey all,
I have problems with tranfering calls from asterisk to a alcatel pbx via a sip trunk.
phone is ringing, so notification is working. but after answering, the no conversation is possible.
I had a closer look to the logfiles and I saw that the alcatel is announcing back sip error “420 - Bad Extension”.

Has anyone an idea where I should have a look at?
Do you need any more information?



Does nobody have an idea?