In Google’s own words
Free Google Voice is a personal number, not a business line.
Google describes the free plan as “for personal, consumer use.” Its rules ban bulk and automated texting, and Google can take back a free number if you don’t make calls or send texts for 3 months.
When Google reclaims a number, you “can’t get calls or texts” on it, and you have 45 days to get it back. Google’s $10 plan sells the fix in one line: “Your number will not be reclaimed due to inactivity.”
With GBPN
A GBPN number is a business line from day one, and it stays yours as long as you’re a customer.
More proof (1)Less
“After your number is reclaimed, your messages are still in your Voice account, but you can't get calls or texts.”