Voice covers the phone side of the business: the numbers you use, the calls that came through them, what was said, and what was left on voicemail.
Numbers
Numbers can be provisioned, updated, and taken out of service. Each one is a separate line you can point at a separate purpose.
That separation is what makes the rest of it useful. One number on the van, another on a paid advertisement, a third on your website, and the question "which of these is producing calls" answers itself — see marketing attribution.
Some numbers can be set up to answer automatically. Where that applies, the call is handled and recorded rather than ringing out.
Calls and transcripts
Every call is recorded with its outcome, and a transcript is available.
The transcript is the part worth using and the part nobody uses. A month of transcripts on quotes you lost will tell you why you lost them more honestly than any survey, and it costs an hour to read.
Two things they reliably show. What customers actually ask, in their own words, which is what your website and your quotes should be answering. And how the phone is answered when you are not listening — which is either reassuring or the most valuable hour you will spend that quarter.
Voicemail
Messages are collected in one place rather than sitting on a handset.
Response time on a voicemail is the single most controllable factor in whether that caller becomes a customer. An unreturned message is not a lost lead so much as a lead handed to whoever calls them back.
Where the numbers matter
Call volume is a marketing figure and answer rate is an operations figure — they fail for different reasons and are fixed by different people. See CallRail for the tracking side and close rate for what happens after the phone is answered.