The HAI cellular module will work with an AT&T GO Phone Sim card. A call will cost you about $.25, plus you have to keep the card renewed every month or so at $15 per renewal.
We did testing of the cellular module from the origonal manufacturer in the far east. It works quite well. It makes a direct call to the central station without going through a backend server. It can also send SMS messages to a cell phone upon alarm.
You can connect your house telephones to it and call out from any house telephone or if your landline fails it will use the cellular network to make outgoing calls.
You can call into the unit, but it does not have a ringer circuit to ring the telephones unless the manufacturer has added it to the cellular module which we requested.