if i call from outside (isdn) after the number of rings choosen, the pbx hangs the line, if i do the same from an internal phone, the answering machine kicks in successfully

this happens to either my 2 external isdn lines configured to ring only on two separate internal extensions (100 and 200)

i tried to reboot it but nothing..

Try downloading the 3.5.4RC5 from the website. I don't have that setup with ISDN, but it's working for me with incoming calls delivered via SIP.