How to split hybrid two-way audio line?
Simply driving the line takes an amplifier and a coupling capacitor, and simply monitoring the line takes a coupling capacitor and an amplifier.  However, the received signal will include the transmit signal.  Keeping the tx signal out of the rx signal is called sidetone cancellation, one of the main functions of a hybrid.  The transformer-based telephone hybrid is over 100 years old, but solid-state versions are much easier to deal with.

Here are two concept schematics from National Semiconductor.  The first is the bare basics: a driver, a receiver, and a subtractor.  The second has a few more parts to compensate the signals for the fact that the impedance of a phone line is not a pure resistance.  It has inductance and capacitance that the line drivers does not.  Compensating for them makes the sidetone cancellation more complete, especially at higher frequencies.

In your case rather than a direct connection or an interface transformer, one coupling capacitor should be enough - IF - it is ok for your interface circuit to share the intercom line ground.  Note that knowing the impedance of the intercom line is an important starting point, but you can start without it and adjust later.