Yes all works fine when not launching the setup with asterisk the MGW can route calls just fine. Below are the configs for sip-custom-contexts.conf and extensions.conf
[GSM]
type=friend
host=127.0.0.1
dtmfmode=rfc2833
canreinvite=no
disallow=all
allow=gsm
context=gsmsubscriber
port=5062
extensions.conf is quite large so i will omit all that was there by default and included is what i have added directly to the end of the file
[gsmsubscriber]
exten=>_xxxxx,1,Dial(SIP/GSM/${EXTEN})
exten=>_XXXXX,n,Playback(vm-nobodyavail)
exten=>_xxxxx,n,HangUp
sip.conf is again default with just this library include at the end of the file
#include sip-custom-contexts.conf
Thanks for the assistance
nat=yes