With a little help from my friend, I have obtained my mobile phone number. And yes, I just got a number. You see, I still had my old (unlocked) T Mobile phone. Nothing fancy, just a phone which makes phone calls (and texts). The guy at AT&T store (not an agent/dealer) told me that their sim card should work in any 3G or 4G phone. So we tried, and sure enough, it works. The sim card was free. (I believe the sim normally costs $6.95.) $25 credit, and I am up and running.
So if you are coming to the States, and you have an unclocked mobile phone (3G or 4G), bring it along, as chances are you will be able to get a go-phone sim card at an AT&T store. Credit of less than $25 expires in 30 days. $25-$75 you have for in 90 days. $100 expires in one year. There are several plans available. You may choose to pay for minutes as you go, or a fixed amount of $2 per day - only on the days you use the phone.
A US address and phone number will be required, but it seemed they were not too fussy as to whose. Even the hotel address and phone could be acceptable. Your choice of number, with your choice of area code.