A SIP phone uses Session Initiation Protocol to set up and end calls over an IP network. It can be a physical handset that connects to your network, or a software app on a computer or mobile device.
SIP phones plug into any SIP-compatible service, so a business is not locked to one vendor's hardware. They carry the standard VoIP features: transfer, hold, conference, and presence.
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.