Another success story. Ported AT&T U-Verse voice number to T-Mobile prepaid to GV. I don't think it is important, but note that the number was originally with SNET before porting to Sunrocket (RIP) to Packet8 (now 8X8) to U-Verse. I have had this number for >20 years.
First, use the T-Mobile web based prepaid SIM activation, which allows entry of a porting request. No need to call anyone, it is all done online. Note that the AT&T account number is available in the U-Verse online statement, it is NOT the phone number or anything like it. That port took about 1 day. Then logged into GV and initiated the port from T-Mobile. Use 1+ phone number for the account number, and the PIN you used online when activating the T-Mobile SIM. That port took about 1 day. I started on Monday morning, and on Wednesday morning, my number has ported from AT&T to T-Mobile to GV. Awesome.
Now to call Comcast and establish internet service, then call AT&T U-Verse and cancel the whole bundle (internet, tv, phone). With Netflix and Hulu Plus and local channels on antenna, all I need is high speed internet. Voice and TV services are overpriced and obsolescent.