What the caller does
A caller who reaches voicemail has three choices: leave a message, try again later or ring someone else. People with an urgent problem usually ring someone else.
A caller who is answered gets a person to talk to. Their question is answered, the job is booked, and they stop looking.
What you do afterwards
With voicemail, the work starts when you finish for the day. You listen, ring back, often get their voicemail, and try again.
With an answering service, the call ends with the job in your diary and a summary on your phone.
When voicemail is fine
If your callers are existing customers who know you will ring back, voicemail can be enough. It is new work, from people who do not know you yet, that voicemail loses.