The system uses a 24V transformer. I am not sure of the current draw.
I have extra 24 gauge wires available from an old phone system and I'd
like to use them (I won't have to rip open any walls.) I'm guessing 24
gauge is "probably" okay but not the best. (I can't pull the old phone
wires and install a new 18 gauge (???) cable, there are too many
staples along the path.)

What is the normal wire size for this situation?