There are lots of emergency telephone systems on the markets such a **broken link removed**. You can try googling for "fire telephone".
Curent thinking can do specialist stuff like systems to put several audio channels down one pair of wires or optical fibre systems for very long tunnels.
I havn't done much work with emergency telephones but if you buy an ready made system it's usually as simple as connecting the wires to numbered terminals. I'v seen a few custom panels with LEDS that light up when someone is calling from an emergency phone so if you just want your SCADA system to know when a phone is in use it should be easy enough.