asterisk users - Nov 2004 - Cisco 79XX phone using dhcp can call out but not in

All,

I got my cisco 7960 phone to work. Two issues I had was correctly setting up
the tftp server for access and the fact that I had to FIRST use a 
version 5.1.
I could not go directly to verison 7.3. So the phone is working.

I can make calls out on the phone - but I cannot call into it. I get
Unable to create channel of type 'SIP'

The phone is setup for DHCP.

sip.conf has
[510]
type=friend
dtmfmode=rfc2833             
username=510
secret=XXXXX
disallow=all
allow=ulaw
allow=alaw
host=dynamic

What is the magic part I am missing.

I have setup other phones gransdtream but not with DHCP.
They all had static IP addresses and work fine...

Thanks,