I'd give pro intercom a call. They have a couple of boxes, the AD903 being the one I'm familiar with, that converts party line to four wire audio, and also the AD920 that convert party line unbalanced to four wire intercom. The AD920 can convert signalling as well to match four wire comms, but what I don't know is if you can just use a pair of AD920s back to back.
Not a cheap solution, but an off-the-shelf solution.
Its a pity the designers and manufacturers of the AD903 did not include a call signal passthrough using audio signalling.
Yes, you can run two 4W converters back to back, that is what they are meant for. You will however have to remotely power each end or be limited to the small number of stations that can be powered by the converter, assuming it can provide power, most can.
Mac
