We're using Asterisk and I have an extension setup to use on my N900. The extension registers fine and I can place SIP calls from my N900 to another extension and there's two way audio.

However, if I call from the other extension, to my N900 extension, the N900 never rings. Asterisk shows the call being placed but it never shows up on the N900. I don't see any errors in the Asterisk logs either.

Any thoughts?

You might try canreinvite=no. If the call fails and it's a routing issue, it shouldn't show up in the Asterisk logs iirc, because the call isn't actually going through the server. The caller is simply being sent a REINVITE. Placing it on the N900's peer config in sip.conf should be enough to make it work if that's the cause but you might try both extensions just to be sure . Try nat=yes too.

My config was this:

[702]
type=friend
context=internal-menu
secret=sec
host=dynamic
canreinvite=no
user=702@host
tcpenable=yes
transport=tcp

EDIT: This was for an N900 that could connect from external and internal host sometimes through NAT.