hbiss: I hope you don't scold me too much for this, but...
When I ran network cable throughout my house, I also did the phone and intercom wires in Cat 5. My phone company uses Cat 3, but Cat 5 worked well for me, no noticable difference (for better or worse.) My intercom system was meant to use standard 4 conductor telephone wire, so I just used Cat 5. I bought a 1000' box of it, made up patch cables and everything. My thermostats and doorbell, I use thermostat wire (don't know the proper NEC name). I agree that you shouldn't use it for many things people do (speaker wire, LV lighting, etc.)
trollog: Is the cat5 already laid out? I have seen some com systems that use a shielded speaker cable, but I think most use a 3 or 4 wire cable, unshielded, 24 gauge. I forget what cat5 is, but I think it's 24, so I'm pretty sure you'd be alright as far as safety and it actually working. As far as NEC, I have no clue.
But think of this: Today, the customer might want an intercom, 5 years from now, they might want the whole doorbell/camera/videoconference/IP camera/etc. thing that will come along one day. If it's a network device, cat 5 could be viewed as planning for the future.
But, as always, you should follow manufacturer's instructions... sure, they might say use 3 conductor #24, but if you use cat 5 and something goes wrong, it could void warranty or stupid stuff like that.
Just food for thought... think of it as a buffet, take what you want, come back for seconds if you wish.
