Extension vs Trunk

I need to know how to register a SIP GSM ATA device as a trunk port for both inbound and outbound calling using Aserisk dial plan so extension calls beginning with say 9 are auto routed over this GSM trunk port.

The SIP GSM ATA has basic SIP config info
SIP Server
User name

I am confused do I need to enter certain info in the GSM ATA registration fields that will tell the Asterisk server this is a Trunk port and/or do I need to set some config fields in Asterisk Admin that designate the device as a trunk port.