sip_general_custom.conf = strangeness

First off, thanks to everyone who's had the patience answering my questions and not telling me to screw off.

Okay, here is a strange thing I have found using the System Status Version: 2.6.2.2 of trixbox ce

I have a Nokia E51 with a built in SIP client that I have connecting to my trixbox server behind a good firewall (SPI, NAT etc.). I have always had a royal bitch of a time getting it to have two-way audio and two way calling while traversing the NAT firewall. it connects, dials, just NO sound.

I read and read and read and eventually found the problem, or at least one of them and I'm sure there is a fix but I'm posting this because I'm sure someone else is scratching their head.

This is what DOES NOT WORK when adding NAT entries to the file "sip_general_custom.conf" . (call will dial, connect, but NO audio either way)
externhost=yourdomain.dyndns.org
localnet=192.168.0.1/255.255.255.0
nat=yes

This is what DOES WORK when adding NAT entries to the file "sip_general_custom.conf" . (call will dial, connect, and two-way audio)
externip=xxx.xxx.xxx.xxx
localnet=192.168.0.1/255.255.255.0
nat=yes

It seems that my asterisk server has trouble resolving my dyndns domain. I can connect to trixbox and dial other extensions using the host name method, but i simply don't get audio... when I switch to an IP, voila, audio. Any ideas how I can actually use the host name? I hate hardcoding IP's... no doubt they change when you are away from your computer.

Thanks again