I used T-Mobile's non-data pay as you go service before getting a Galaxy Nexus phone. After getting my new phone, I went into my local T-Mobile to get the $30 plan. They didn't have a clue what I was talking about. After going home, a bit ticked off, and looking at the T-Mobile website again, I figured out that I needed to go to either Walmart or sign up online through the T-Mobile website. The T-Mobile stores don't offer this plan.
I wanted to keep my old number, so I went to Walmart so that I could speak to a human. I took the SIM out of my old phone and tried activating it under the $30 plan. No dice. It is for new activations only, so I needed to buy a new SIM card. Walmart didn't have a SIM card (unless you bought a phone), so it was back to T-Mobile. While trying to activate with the old SIM card the tech from T-Mobile said it was going to cost $6. When I was at the T-Mobile store they said their tech didn't know what she was talking about, and the cost was $22. I complained briefly, but at this point I just wanted to get the thing working, so I paid the $22 bucks. (Note: if you have an existing T-Mobile SIM, tell them you were moving it to a new phone and dropped the SIM and lost it. They will give you a new one for free. I found this out afterwards.)
Back to Walmart. The guy at Walmart calls T-Mobile again and this time I get the phone activated, and they also took care of transferring my old phone number to the new SIM. I was told by the T-Mobile tech that it would take 24 hours before it would start working, but I checked 30 minutes later and it was all set.