Oh lordy, the phase-splitter intercom; first one I ever built, waaaaay back... An alternative is to drive the intercom line with a Howland current source. You don't have the problem with the collector resistors paralleling up as you add stations on the line, and the sidetone cancellation is much better because there are no reactive components introducing different phase shifts and frequency responses into each station. Built a bunch of them back in the 70's.
There's another intercome style floating around that you can consider. It uses the LM386, so better audio and more volume for less power than Colin's second circuit. Also, it has a common base amplifier for the speaker-as-microphone. Both stations listen to the common signal line all the time, and each station is push-to-talk. No local feedback, no phase-spliter to adjust, but also not full duplex. If anyone's interested, I'll fish through the archives for the schematic, and I'm sure I don't remember where I got it so I can't credit the source.
ak