Ok, excellent, thank you Col Ronson, deeddawg, & skimcpip for your help! *I've revamped all your helpful info to jive w/how my brain works (& which may help other "oldsters" out there), as follows: *
1. *All iPhone Models' SIMS:
http://support.apple.com/kb/HT5554
--REGULAR: *iPhone Original, iPhone 3G, and 3GS take regular sized SIM cards.*
--SMALLER; *iPhone 4 & 4S take Micro-SIM.
--SMALLEST: *iPhone 5 & 5S take Nano-SIM.
2. *WALMART: *Their SIM-Kit is specifically advertised to work straightaway with the desired T-Mobile $30-Plan:
http://www.walmart.com/ip/T-Mobile-SIM-Kit/24099996
--PROBLEM: *That Walmart/TMo SIM-Kit is the wrong size, too small (Micro-SIM) for a 3Gs (Regular-SIM).
--SOLUTION.1: *Buy SIM-Resizing-Kit at eBay, Amazon, Walmart, etc., to make above Walmart/TMobile $30-Plan SIM bigger (increase Micro-SIM to Regular-Size-SIM to fit 3Gs).
--SOLUTION.2: *Buy Regular-Size-SIM at Target which naturally fits 3Gs, no Resizing-Kit required:
http://www.target.com/p/t-mobile-si...4586517#prodSlot=medium_1_1&term=t-mobile+SIM**
--a) *But because that Target-Reg-Size-SIM is not "encoded" to work with the specific T-Mobile $30-Plan, you would put it in the 3Gs anyway, but when doing the "self-activation" TMo $30-Plan at TMo website, you would type in the Activation-Code that came with the Walmart/T-Mobile $30-Plan-SIM-Kit rather than the Activation Code that came with the Target-Reg-Size-SIM.
3. *ACTIVATE at T-Mo Prepaid Site online set-up:
http://prepaid-phones.t-mobile.com/prepaid-activate
--Any problems, follow BruiserB's tips further above & here:
https://forums.macrumors.com/posts/17860172/
He wrote:
A) *"Are you sure your phone is SIM unlocked and will work with a T-mobile SIM?" *(Q: Is "Sim-Unlocked" the same as "IMEI-Unlocked"?)
B) *"Is the [Walmart/TMo $30-Plan Sim] IN your phone?" *(NOT IF I BOUGHT THE REGULAR-SIZE TARGET SIM FOR 3Gs, NO; the Target Sim would be in 3Gs: *See "Solution.2(a)" further above.)*
C) *"Is Your phone ON?"
D) *"Are you in a place that you know has T-mobile signal?"
E) *Make "sure you entered the SIM card number correctly."
F)**He*"got a confirmation text within seconds."
THE END.
(Whew! *Think I got it, right? *Feel free to correct any errors in the above process. *You're dealing with non-techie 60+ & 80+-year-old ladies here (me & Mom) who are trying to keep from drowning in the too-fast-moving-for-us Techno Race. *The other problem is sim-swapping in itself, ie, taking apart iphone hardware. *Ugh! *I can't do it but there's a tech-shop nearby, a guy who can do it).
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Asterisks in above post? I did not type them in; not sure why they are there or where they came from. I typed out that post in 3Gs native yellow legal pad, copied & pasted in forum reply window.
All the asterisks are annoying. Sorry about that.