As mentioned, I don't have a HAI C3, so my understanding is purely from research and I may be wrong.
From what I've read, the C3 connects to a M1G as if it is a POTS line. The POTS line plugs into the C3. The M1G thinks it is connected to a POTS line. If the C3 detects that the POTS line is down, it will route all outgoing calls thru the cellular (GSM) connection. That includes calls originated by the M1G and optionally, calls from one of the phones connected to the POTS line. I believe you can also "force" a call from a phone on the POTS line to go over the cellular connection by entering a key sequence before the call. I also think that if the POTS line is down, the C3 will route all incoming calls on the cellular line to the POTS line phones. For example, let's say that an intruder cuts your phone line then breaks into your home. Either the phone line being cut and/or the break-in will result in an alarm call going to the central station over the C3's cellular connection. If the central station is set up to call back to your home via the cellular number, when it does the C3 will route the call to the POTS phone so you can answer it.
I'm a Verizon customer, so I don't have a GSM phone contract. I will get a AT&T GSM pay-as-you-go SIM for the C3, and set up the C3 as the backup line for the M1G, with the POTS line as the primary. That way, all calls to the central station will go thru the POTS line unless that line is not available. That should keep the pay-as-you-go call charges down to a very small minimum. I think the central station sees both lines as "regular" phone lines, so there is no additional charge for the C3 cellular connection.
Ira
