A PRI, Primary Rate Interface, is an ISDN circuit that bundles around 23 voice channels onto a single physical line. For years it was how mid-size businesses connected a PBX to the phone network.
SIP trunking has largely replaced PRI by delivering the same channels over the internet, with capacity you can add instantly instead of ordering new circuits.
Frontdesk gives you a business number answered by an AI receptionist that picks up every call, qualifies the caller, books the appointment, and logs the whole conversation to your CRM.