There are several ways to update your Nutone Intercom System. I think the system you have is a 3 wire system. I could be incorrect but if it is so you have the following options.
1. Get the system repaired from an authorized nutone repair center.
2. Replace it with a system that works on a 3 wire system. There are only two brands out there that have systems that work on 3 wires. That is either Nutone www.nutone.com or www.mssystems.com.
3. Last choice would be rewire the entire house which could become costly depending how your house is wired and built.
Please let me know if you have any questions and I'll be glad to answer them.