[Asterisk-Users] How to best debug SIP registration failure

[George Pajari]

I am having trouble getting SIP phones to register with Asterisk. I know
that the phone can register with FWD and I have used tcpdump to see the
registration packets arrive at the Asterisk server, but nothing goes back.

How should I attack the problem?

What debugging tools exist to try to determine why Asterisk is not
accepting the registration requests?

I have googled for answers but no one seems to have posted a general
approach to attacking such problem, or, if they have, I missed it.

g.