The GSM telephone interface allows access to other telephone users where no landline exists.

It can be used with any ordinary 2W DTMF telephone handset.
In 4 wire mode it can be linked to a sound desk for "phone-in" or into a talkback system for remote communication.

Outgoing dialling can be performed from the handset or via the 4 wire audio using standard DTMF signalling.
Auto mode allows auto answering of incoming calls for connection to a recorder to allow unattended report collection.
Instead of connecting a single handset it can even be linked into a PABX exchange line port to give all personnel access to the outside world.