A couple of ideas:
1) Rather than hard-wired telephone/intercom system, consider a 2.4 GHz wireless system with multiple handsets. Quality on these is very good -- better than typical cordless phones. Some high-end systems offer ability to call from handset to handset (i.e. intercom). Only the base unit needs to be hooked into the phone line(s). The most popular system like this is Siemens, but lots more are coming onto the market.
2) To make sure that you hear the phone ringing, you can use an extra-loud ringer, which plugs into the phone line. You can get these inexpensively at Radio Shack. Another thought is a phone that is designed for people with hearing impairment -- these have ringers built in that are extra-loud and adjust for ring frequency. The best is Walker Clarity -- available in both wired & wireless (also at Radio Shack).