Google Voice is not a "free telephone company".  It is a call forwarding and message management system.  You still need some sort of other telephone service, in case something goes wrong with Google Voice, and to make emergency calls.

Google Voice cannot call 911.  Again, it is not a substitute for full-function telephone service.

Google Voice can only accept ports-in from mobile phone carriers.  You would have to first port the number into a mobile phone carrier,  wait a full week for that port to fully complete, and then port the number into Google Voice, which can take several more days.  You can't do this instantly, in one day; attempts to rapidly "churn" the port between carriers may cause the port to get stuck or fail.

Cost:  at minimum, you would need a prepaid phone, or a prepaid SIM to put in somebody's unlocked phone, a minimum purchase of calling minutes (typically $10) and a $20 fee paid to Google to perform the port-in from the mobile carrier.

Before porting your land line number to a mobile carrier, avoid the disappointment of finding out that you can’t port that number into Google Voice.  Using a desktop or laptop computer’s web browser, enter your phone number into the Google Voice porting dashboard:  https://www.google.com/voice/porting. There are two different error messages it may return. It will always report that your carrier isn’t supported.  This is expected, since Google Voice won’t port numbers in from land line carriers. However, if it also tells you that your area isn’t supported, stop now.  Your number cannot be ported into Google Voice, and there is no workaround.

You must wait a full week after you port your number from the land line carrier to the prepaid mobile carrier.  Make sure that both outbound and inbound calling and text messaging work on the mobile phone before taking further steps.

You must contact the mobile carrier and specifically ask them for your account information.  To minimize the chance of the carrier trying to retain your business and/or question why, I suggest saying:  "I need to port my number out of (this mobile carrier) to a land line carrier.  What is the account number and PIN I need to submit to my new carrier, so that (this mobile carrier) will accept the request?"  This information is often not displayed on the carrier's web page.

You will need to submit the porting form on the Google Voice web site, which may not initially have a spot to enter the PIN, wait for it to fail and come back with a note complaining about a missing PIN, and then you can reopen the porting form, which will now have a space for you to type in the PIN.

You must purchase enough prepaid calling minute credit to be able to make and answer a few test calls and texts on the mobile phone, and to answer the automated verification call from Google.