Your SIP URI is
123456@voxalot.com (substitute your VoX number for 123456)
You can also use these alternatives:
-123456@xx.voxalot.com (where xx can be us, eu or au depending on whic server you're using).
-*010123456@voxalot.com
Now if the GSM gateway can send calls to SIP URIs, incoming calls to the gateway can be passed to VoXalot.
The second part, about passing calls from VoXalot to the gateway... does the gateway have its own SIP URI...or maybe does it allow you to register with a SIP Provide to receive calls on the SIP provider and pass them on to GSM...
What's the grand scheme of things you're trying to achieve...maybe I can be of more help if I understand what you're trying to get at...