Asterisk and NAT

Hi, I’ve been googling a lot for the following question but could not find any answers. It seems like not a lot of people are using the same setup I have.

I have my asterisk server running behind nat. But my router has port forwarding setup for a range of RTP ports and SIP port towards my server.

I have a phone behind another nat, but on that router, I also setup port forwarding for SIP and RTP towards the phone.

So I believe that I should not have any problems with ports. So on my asterisk server, I set externip=blah in sip.conf.

I am wondering if I really need to use a STUN server from the phone. Is there a way I can just tell asterisk to use the ports specified in the SIP messages but ignore the IP address and send it to the address it came from?

Am I right in assuming that the only thing I need to worry about in a setup like that is the IP addresses in the messages and not the ports?

Thank you.