I found this on Google:
”You can't port a landline phone number directly to Google Voice. Instead, you need to port your landline number to a mobile carrier. Once your landline phone number is recognized as a mobile number, then it can be ported to Google Voice. The one-time porting fee to Google Voice is $20.”
I saw other articles regarding forwarding calls from a landline to GV, but this is all I really found regarding porting a landline number.
I ported my parents landline to GV. I had to go though a middle-man..Bought a really cheap pre-paid at&t sim, ported to that, activated, then ported to GV the next day.
They are still using their landline with an ObiHai. No phone bill.