Interfacing to a third party analog intercom system can be a little tricky only because of the wide variety of possible setups out there. However the main objective is the same, insert Unity's audio into the existing system in a way so that it does not feed back into itself.
To get started you will need to purchase the Unity I/O license to activate this capability. You will also need a multi-channel USB audio I/O device, almost any PC audio interface that works with a Mac will be fine. The main thing to keep in mind is 'don't send Unity back it's own audio'. For example, if you send the audio from Unity channel 1 out of your PC interface on output 1, make sure it does not appear at any input of the interface. If you do, you'll get a bad echo.
Some people have solved this by sending Unity channel 1 information to the program feed input on their existing Com system, and their program feed is sent into channel 1 on their 2-wire system. They then talk on channel 2 of their 2-wire system. They listen on both channels. Channel 2 of the 2-wire system is sent back to Unity. The problem with this situation is that you tie up both channels on your existing 2-wire system. If this isn't a problem, then it's certainly a workable solution.
A better situation is to use a 2 to 4 wire audio nulling interface device available from several manufacturers, basically all this device does is simply make sure that most of the audio sent into the system is removed when received back. While not a perfect solution to the digital echo problem, it is definitely a workable solution to maintaining both channels of a legacy 2-wire system.
Below is a helpful drawing illustrating interfacing Unity into a third-party analog audio system