[Freeswitch-users] How to know inbound call arrive from which gateway?

Sorry this maybe a very simple question but I checked Wiki/Dialplan without finding an answer. How can I know inbound call arrive at which sofia gateway in dialplan? My purpose is to add a prefix to all inbound call from a certain gateway. I cannot use the "destination_number" built_in variable for this purpose, as in my case DID bind to a specific gateway hardware is often changing. Is that another varialbe to be used, eg. a gateway name etc? /Alex