Most unlocked phones from the last few years work on Flex Mobile. There are three things to check, and all three take under a minute.
Check 1: your phone is unlocked
A locked phone only accepts service from the carrier that sold it. On iPhone, open Settings, tap General, tap About, and look for Carrier Lock. If it says No SIM restrictions, your phone is unlocked and you can move on to the next check.
There's a full guide for Android and for what to do if your phone is locked. See the related articles below.
Check 2: your phone supports eSIM, if you chose an eSIM plan
On iPhone, open Settings, tap General, tap About, and scroll down. If you see an entry called EID, your phone supports eSIM. You also need iOS 14.1 or newer, which you can see at the top of the same screen.
On Android, open Settings, tap About phone, and look for EID. On most phones you can also open your dialer and enter *#06#, which brings up your IMEI, the number that identifies your phone, and on eSIM capable phones your EID underneath it.
Not every phone in a model line has eSIM. US versions of the Galaxy S20, S20 FE and S21 don't, even though the same models sold elsewhere do. The EID check is what settles it, not the model name. There's also a full device list in the related articles below.
Check 3: your phone works on the network
Flex Mobile runs on a nationwide 5G and LTE network, and most phones sold in the US in recent years support the bands it uses.
You can also check coverage where you actually live and work. Enter your zip code on the Flex Mobile coverage page and it shows voice, 4G LTE and 5G availability separately for your area. Your zip code is checked again at checkout.
If you are buying an eSIM plan, each plan page on the Flex Mobile store has an eSIM compatibility check built in. Use it before you buy and you will know for certain.
If your phone doesn't pass
A locked phone can usually be unlocked by the carrier that sold it, and the unlock guide below covers the major carriers. Contact Flex Mobile support if you're not sure whether yours is locked.
If your phone has no EID, it can't take an eSIM, so it isn't one you can use to join Flex Mobile. You'd need a phone that supports eSIM.