Hello Markosjal,
No offense meant and thanks for trying to help but. Did you really read what the problem is about? Nobody is talking about authenticating or something. So please read and understand what this thread is about.
The point is that HT503 can transmit the Caller-ID also in a different SIP Header fields, one of them being “P-Asserted-Identity”. On some forums they say that this is more reliable and so I tried to configure freepbx/asterisk to make use of them. As you see the above sultion which looked quite promising doesn’t work, I have no clue why but then extension for incoming calls from the HT503 stops working.