Can Microsoft Teams be your whole business phone system?
Yes. Microsoft Teams can make and receive ordinary phone calls when each user has the required Teams Phone licence and the organisation connects it to the public phone network through Calling Plan, Operator Connect, Teams Phone Mobile or Direct Routing.
Standard Teams already handles app-to-app calls with colleagues and, where admins allow it, external Teams users. Calling ordinary telephone numbers is different. You need Teams Phone plus a route to the public phone network: Microsoft Calling Plan, Operator Connect, Teams Phone Mobile or Direct Routing.
For a business already centred on Microsoft 365, that can be a sensible route. Staff use a familiar sign-in, and calling sits alongside chat and meetings rather than in another product.
The setup depends on the route you choose. Calling Plan and Operator Connect are more managed; Direct Routing gives more control but needs more telephony configuration. Compare the Teams licence, carrier service and administration together, because the phone service is not automatically included in every Microsoft 365 seat.
For the fuller side-by-side, including where Teams Phone still wins outright, see Olatti vs Microsoft Teams Phone.