The only limits on 1A2 are the fusing/power supply ratings, and of course the number of intercom dial codes. As for the protectors, don't go with something low-voltage rated because ringing current will trip them. You will need standard ones, typically rated at 235 volts. Anything lower will result in miscellaneous problems, specifically ring trip.
As for the Melco units I had mentioned, they are about the highest quality you can get, and yes, they are solid-state (opto-isolated). They are in a blue plastic wall-mount enclosure with screw terminals for the connections. Melco was a HUGE manufacturer of 1A2 intercom systems for Bell companies and probably the biggest name in service observation units. These cost a lot more than the ones you located, but if it works then go for it! Again, if you need one, just let me know and I will send it out.
