I was able to transfer my landline number which was on AT&T to Google Voice using a T-Mobile sim card. So it is possible. They are different carriers. You won't be able to have 911 on Google Voice unless you pay for a plan on
Obitalk.com that gives it to you. But you can still transfer your landline number.
1) You MUST have your account information for your landline. Including a transfer pin required to migrate your landline number over. For AT&T if you look at your billing statement. It's the set of 4 numbers at the end of your account number. It can also be the set of 3 numbers at the end as well. So give them both, just in case. (But to it's usually 4 numbered one). If in doubt call your service provider. Tell them you want to transfer your number and you need the transfer pin.
2) Have an unlocked phone. (You can call your carrier to have them give you the unlock code for your phone. It's free) OR you can purchase an unlocking code on the internet.
3) Buy a T-Mobile sim card. I used this one since it comes with 2. (
https://www.amazon.com/gp/product/B07MTTCLBW/ref=ppx_yo_dt_b_asin_title_o06_s01?ie=UTF8&psc=1)
4) Put the sim card in your unlocked phone.
5) Call T-Mobile NTC Department. 1-877-778-2106 or (1-800-937-8997)
6) Tell them you want a temporary number on a sim card you just purchased. Tell them you want to migrate your landline over to this new sim card.
Google Voice requires you to AUTHENTICATE your temporary phone number. So if they offer you the (Internet $0 Dollar Plan) for the temporary number
it won't work! You need the ($3.00 Complimentary Data Plan for 30 minutes). Or at least the cheapest possible thing to get one single phone call. That's all you need.
7) Remind them you are transferring a number. Give them your account information from your landline. Including your transfer pin.
VERY VERY IMPORTANT!!!!
Ask T-Mobile to give you your new ACCOUNT NUMBER (not telephone number). Plus have them setup a temporary pin. Like 1234 or whatever you want. You will need this to transfer to Google Voice!
Once everything is done your phone should have your landline number transferred in about 24 or 48 hours. WAIT for it to fully complete.
9) After 24 to 48 hours. Go to (
https://www.google.com/voice/b/0/porting/?gsessionid=qZX-kJ6G3xvgcoWRXbPLDTKazdJsMXJH) and see if you can transfer your number yet. Don't freak out if it still No even after a few days. You can still transfer it, with it being no.
10) Be patient and go to (
https://support.google.com/voice/answer/1065667#xferin) and follow the instructions under "Port your Number". The charge is $20 dollars.
11) After 24 to 48 hours your landline number will be transferred from your T-Mobile sim to Google Voice. Tada!