; add this to general 
nat=yes
canreinvite=no
externip={yourip]
localnet=192.168.1.0/255.255.240.0

[4888]
context=imt
type=friend
host=dynamic
username=4888
callerid="Andrew @ Canada" <4888>
dtmfmode=rfc2833
mailbox=4888@imt,4888
nat=no