That's a tough one. As mentioned above if the 2N can output a contact that can trigger the system to ring. If the PBX already has a doorstation card installed there is nothing else you would need. You would just wire the 2N relay to card as if you were wiring a doorbell button to it.

On the more advanced side, something like an ATA would work. However, these are typically used in the opposite way for example connecting a fax to an IP phone system.

You could always upgrade the phone system. The NS700 would handle the 2N directly as a SIP extension, and the existing phones would work as well as long as hybrid line cards are installed.