I am in the process of planning and designing a device, which is actually two devices, a base station, and a small pendant (which will be larger than I would like during prototyping).
The base station will have a temp and humidity sensor, a GSM & GPS module, along with a few other things I haven't decided upon yet.
The pendant will have a way of communicating with the base station (yet to be decided, because of the distances potentially involved), a flame sensor, a shock sensor, and a few other little bits.
One thing which has been suggested, on the base station, however, is to have the system connect up to the landline network (using a passthrough, so that there isn't a dedicated socket required), to call out and play a recorded message. There may be several different messages which may be played, so I guess a micro-SD card will need to be added in at some point, potentially using a joint ethernet and micro SD card shield.
So, would it be possible to do the passthrough using the telephone system? If so, what would I need to get it done? I've seen there may be some complications regarding voltage, and things like that.