To get a Google Voice number, sign in at voice.google.com with a Google account, search for a number by city or area code, and verify it with an existing US phone number. It is free for personal use in the United States. Here is the full process, plus what to do if Google will not accept your verification number.
Requirements
A personal Google account (Gmail).
To be in the United States. Personal Google Voice is not offered in most other countries.
An existing US phone number to link. Google sends a code to it to prove you are a real person.
How to sign up for a Google Voice number
Go to voice.google.com on a computer, or install the Google Voice app on your phone, and sign in.
Accept the terms and tap Continue.
Search for a number by city or area code (for example 212 or Chicago) and pick one from the list.
Tap Verify and enter your existing US phone number.
Type in the 6-digit code Google texts you. Your Google Voice number is now active.
You can now call and text from the app, the website, or have calls forwarded to your linked phone.
Why Google Voice will not accept my number
Google requires the linking number to be a real mobile or landline number in the US. It usually rejects:
Other VoIP numbers (TextNow, TextFree, Skype numbers).
Numbers already linked to another Google Voice account.