Thanks for the responses. I originally posted this question because time was an issue. I wanted to test the Straight Talk SIM in my T-Mobile phone before T-Mobile's "buyer's remorse" period ran out and I thought I had to have the phone back in their hands within 14 days of when they shipped it. It actually has to be postmarked within 20 days, so I have a little wiggle room.
Straight Talk says I can put one of their T-Mobile compatible SIMs in my phone and it will work, and I will get 4G LTE. They also said if I bring my own phone from T-Mobile and buy a new service plan I'll get the 5gb of data they're offering. (Frankly, that extra data is one of the reasons I'm switching back from T-Mobile. My new phone is going through data like crazy even though I keep it turned off when I'm not using it and spend most of my day in WiFi areas.)
So here's my plan: I've ordered a Straight Talk SIM for my T-Mobile phone. It should be here in a day or two, before my remorse period runs out. Straight Talk says I can transfer my existing number and plan to the new SIM and then back to my old phone. Assuming Straight Talk works as advertised in my T-Mobile phone, I'm going to cancel my T-Mobile service after the remorse period runs out. At that point, yes, they will lock my phone. The remaining balance on the phone will be on my last bill. After that bill is paid in full, they will unlock my phone from their network. Then I will move my Straight Talk number back to the new phone and leave it there. If the new Straight Talk SIM doesn't work, I'll return the phone to T-Mobile before the remorse period ends and just pay the restocking fee.
This all sounds kind of shady and underhanded, but 1) I signed on with T-Mobile intending to stick with them, and 2) I did not know Straight Talk offered 4G LTE or 5gb of data until after I'd decided to cancel with T-Mobile. I used the phone for over a week and tested every aspect of both the phone and the service, so I feel like I made an honest effort. My old phone only uses about 100mb of data a month so I thought 1gb would be plenty, but the new phone has already used a little under 10 times the data in the same time period. (Since June 14, my Straight Talk Verizon S3 has used 24mb of data. The S6 on T-Mobile has blown through 134mb. I know some of that was probably from starting up a brand-new phone and account, but it's steadily eating through about 10mb a day where my old phone uses around 1-2.)
The reason I switched from T-Mobile in the first place was because their "excellent 2G coverage" in my hometown rendered my smartphone unusable. T-Mobile used to be king in this town, and everybody I knew used them until we all started having the same problem--our data was slow to nonexistent and even on WiFi, we were missing calls and texts--and T-Mobile themselves told me that once you reach your data limit for the month, 2G is what you get throttled back to. And I expected and was willing to pay a little more for T-Mobile service, but after insurance (which I know is optional) and fees (the same fees Straight Talk pays), it came out to much more than I was expecting. $20 more a month for a third of the data and the potential to have my new phone become even more useless than my old one, that's just a deal-breaker for me. (That's not including the price of the phone. With the EIP payment, my monthly bill skyrocketed from $46.67 on Straight Talk to $94 on T-Mobile!)
I will say that I have no complaints about T-Mobile's signal strength, reliability, speed, or customer service. I would like to support them financially because I support their decision to finally upgrade their speed in our area and I think that's the way free enterprise is supposed to work.
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