Google Voice will only allow you to port cell phone numbers to their service (don’t ask ME why — seems stupid, but that’s their rule) so if you want to port a landline number, you first have to temporarily port it to a cell phone provider, then from there port it to Google Voice. Most of the published methods that I have seen for doing this involve paying out some small sum (usually around $20) to get a “disposable” cell phone (so after adding the $20 that Google Voice changes to do the port you are out almost $40), however I just stumbled across a thread that suggests it may be possible to do it for free, IF you have (or can borrow) an old Verizon or Page Plus cell phone that’s not currently being used. Note this may not work in all areas (there are still areas of the country where Google Voice can’t port numbers) and I don’t guarantee it in any case because I haven’t personally tried it, but if you’ve been thinking of porting your landline number to Google Voice, this MIGHT save you a few bucks:
Again, although it’s not clear from this thread, Google Voice will still charge you $20 to do the port, but if you can get this to work it could save you some money. Note that if you are served by some Podunk (independent) telephone company there’s a good chance it won’t work, so keep that in mind if losing your number would be a major catastrophe for you.