A phone extension is the short code (often three or four digits) that reaches an individual or department inside a business phone system without dialing a full number.
Extensions live in the PBX. Callers reach them through an operator, an IVR menu, or a direct DID number mapped to the extension.
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.