I've quickly glean off the site Jpeyton referred me to (howardforums) - it's an understatement that I was pleasantly surprised to learn of so many options. I could have saved thousands of dollars! Even AT&T has its pay-as-you-go plan called (AT&T GoPhone)! Of course AT&T rep never mentioned it to me when I discussed my plans back then. Though to be fair I was getting a new phone with a 2 year contract, so such a plan was a non-starter, I suppose.
Thanks to everyone, now I know I can unlock my iPhone 4 and use whichever service that I decide on according to my needs. Oh and I now understand the difference between cellular network and the Internet! Thank you again for enlightening me. I still don't know what the heck 3G, 4G, LTE, HSPA+, etc., stand for. (MMS/SMS still confuse me as well. What do they do exactly? I remember the days I could use text messages without either of them)
The reason I thought of cellular network as the Internet was because I could make phone calls disabling "Cellular Data" and "Enable 3G" options in my iPhone. Now I have a better understanding, and I suppose "Cellular Data" is different from "Cellular Voice."
I checked out Google Voice a little more, and it offers a service that let me keep my phone number forever for $20. Is it worth getting, just in case? Now that I am going on my own without an ongoing contract.
Choosing among these prepaid plans, I think I really do not need Data plans if Data plans can simply be covered by Wi-Fi. If that is the case, how much data a plan offers or how fast one is should be irrelevant to my choice, correct? All I need is a voice plan - so what matters is rather a stable coverage, not fast network (LTE?) or how much data a plan offers. I would like to be sure about this.