Most small businesses run the phone off somebody's mobile. It works until it doesn't. One person holds the number, and when they're on a job, driving, or on holiday, the caller hears nothing and rings the next name on the list.
A phone system fixes the part a mobile can't: what happens when you don't pick up.
One number, more than one person
A shared number rings everyone who should hear it. Whoever is free answers. Nobody has to be the only person who can.
The call goes somewhere when you're busy
Out of hours, in a meeting, hands full — the call still lands. A greeting, a menu, a receptionist that takes the message. The caller gets an answer instead of a ring tone.
You can see what came in
Every call and text in one place, with who rang, what it was about and whether anyone got back to them. A personal mobile keeps none of that.
The number stops being personal
It belongs to the business, not to a handset. Staff change, phones break, and the number carries on. You can hand it to whoever is on shift without handing over a SIM card.
It costs less than the calls you miss
One missed call from a customer who was ready to buy usually covers the month. That's the whole calculation, and it is why the phone system tends to pay for itself before anyone gets around to measuring it.
You don't need hardware or an engineer for any of this any more. A number, a browser, and the people who already work for you.