I'm happy to report my POTS number port from AT&T (formerly SBC/Pacific Bell) to T-mobile and then to Google Voice was successful. I only ran into two hitches.
The first was with T-Mobile. The initial person I spoke with (before being sent to porting) said I needed my account PIN to port my number. I told him I didn't have one, that it was a landline, and he said to call AT&T and get it. I did and was told there wasn't a PIN associated with my account (but I could add one - no thanks). I called T-Mobile back and was connected with the porting department. When asked for the PIN I just said there wasn't one and the port person said no problem. All I needed was my account number and my name and address EXACTLY as listed on my bill. The port went through about thirty-six hours after the initial request.
The second hitch was I received two "Oops! Issue with your Google Voice Number Porting Request" emails from GV. The first email was immediately after my initial port request and the second was almost two hours after the first email. I know my TMo account PIN was correct but I reset it anyhow (dialed 611 from phone) then submitted the info to Google again via the link provided in the email. I was confused about the request for the last four digits of my social security number because that wasn't info I gave when setting up my TMo Pay As You Go account; perhaps that's why GV emailed twice. Nonetheless, I filled in my last four (it's required) all three times (initial, first email, second email) and the port was successful almost exactly twenty-four hours after the initial port request.
My costs: used T-Mobile phone on Craigslist, $10; T-Mobile SIM, $.099; Google Voice port, $20.00; Obi110, $49.00; for a grand total of $79.99. My monthly AT&T home service (metered, no long distance) was $21.31, so in four months I'll have recouped costs.
One more thing... when I was on the phone with AT&T (landline) I asked if I'd still have 911 service if I disconnected my landline. I was put on hold while the rep checked, then was told yes, it would remain active on my line. Not sure how long that'll last, but I do still have a dial tone.