I have an Android phone with T-Mobile and an ultra-cheap phone with Tracfone. Am I able to swap the SIM from my T-Mobie phone to the Tracone and back again?
If not, am I able to swap the card to another -Mobile phone andback again? I could swear I knew a guy that did that.
Long story, but nothing illegal. 
Thanks.
Sometimes a telephone is tied to a certain provider, but not always, so I’d guess the best way to find out is to swap the cards and see if they work in the other phone.
If you’ve unlocked the phone and it’s a GSM model (AT&T and T-Mobile are GSM, Tracfone appears to be mixed), then go to town.
I unlocked my Samsung S4 via an online procedure (involving entering magic values into specific phone menus and rebooting) prior to a weeklong trip to Scotland this year, I was able to simply swap my GSM AT&T SIM card for an O2 pay-as-you-go card to get 750MB of 4G data (for uploading pics, and using my phone as a WiFi hotspot for laptop/tablet) and 500 minutes of talk time (much more than I needed to make local restaurant reservations and the like) for $about $30. Then I swapped back when returning Stateside. It was great.
Would I need to unlock the T-Mobile phone or the Tracfone? One other thing. When I upgraded the T-mobile phone from a slider to the Android, I had to call in to T-Mobile customer service and give them the new IEM number. Does anyone know if I’d have to do that each time?
Final thought. If I bought a cheap T-Mobile phone could I swap the card and do the IEM stuff myself or would I have to call them each time?
Unlocking the phone(s) seems easiest, but are there any pitfalls to doing that? I just don’t want to have to buy another Android phone.
Again, thanks.
T-Mobile now sells handsets without a contract, which probably means the phone is unlocked from the get-go. “Unlocking” a phone simply means disabling any possible software put there by the carrier to check that you are using a SIM card from their specific carrier for service (to lock you in); it doesn’t mean hacking the phone’s OS in the same way as “Jailbreaking” would for an iPhone.
AT&T at least unlocks any cell phone for free after your contract is up, but the fact is you can unlock your phone any time, just Google online for the procedures.
The key is to make sure the carrier technology will be usable with the service you want to switch to. GSM and CDMA are incompatible. GSM is the more common standard worldwide.
You don’t say what kind of Android phone you have, but I bet Googling the model and the word “unlock” would help you, as it did for me and my Samsung S4: searching on “Unlock Samsung Galaxy S4” => first non-ad hit is to someone’s blog entry on How To Carrier Unlock a Samsung Galaxy S4 in 5 Minutes, which runs through a somewhat arcane 15 step process of configuration menus with magic values to enter. I did these steps as directed, at the end of which my phone was unlocked and SIM-swappable with O2 service in the UK.
I’m pretty sure it’d be just as simple for your Android phone, iPhone unlocking is more of a pain.