Voicemail is the recording a caller reaches when nobody picks up: a greeting, a beep and room to leave a message. Every mobile and most landlines have it, usually switched on by default once the phone has rung for a set time. It costs nothing and asks nothing of you, which is why most small businesses rely on it.
Its weakness is the caller. Many people hang up at the greeting rather than leave a message, and those who do leave one often give a number without a reason. A mailbox can also fill up, after which callers cannot leave anything at all.
In practice
A solo massage therapist reviews her voicemail for a month. Of 52 calls that reached it, 19 left a message and 33 hung up. She recognises 12 of those 33 numbers as regulars; the other 21 are unknown, and some of them were almost certainly new clients. She rewrites the greeting to give the business name, say she is with a client, and point callers to a text or the booking link in her profile. The next month, 9 callers use the link instead of hanging up.
Keep the greeting under fifteen seconds. Say your business name, say when you will reply, and give one other way to reach you. Change it when you are away, and clear the box often so it never fills.
In Glow Line
The Free plan has no phone line, so your booking page link is a good thing to give in your voicemail greeting. On Line and Salon, diverted calls reach the assistant instead of voicemail. See share your booking page.