Long time ATT, SBC, Pacific Bell land line customer in the 415 with a legacy prefix that I wanted to keep. Followed the porting instructions, and my legacy prefix is now a GV number.
Monday – Activated a T-Mobile pre paid sim card on the internet. After verifying the number works incoming and outgoing, called T-Mobile porting department at 1-877-789-3106 and requested a port in from a land line.
Tuesday – At 5pm my ATT land line # was ported to the T-mobile cell phone. Verified calling in and out.
Wed – 7am logged into my GV account and requested the T-Mobile # be ported to my GV. The T-mobile account number is " 1 (area code, number) for a total of 11 digits. Immediately after paying the $20 porting charge. I received an email, saying " there is an error with the port ". Just clicked the link in the email, and a screen will ask for your T-mobile prepaid PIN. Submit it, and you're done.
Thurs – At 7am ( exactly 24 hours after porting request ), I received an email saying, porting was complete. My old ATT legacy number now is now my GV number.
To summarize, activated the T-mobile sim on a Monday, and Thursday porting was completed to GV. So now my Obi receives and makes calls using GV, and I rescued my legacy number!
When GV asks for your T-Mobile account number, it is the " the newly ported in number from the land line , not the number assigned when you first activate the sim card.