I would unlock the phone. Jailbreaking doesn't = carrier unlocked.
Jailbreaking is similar to rooting an Android phone and is unrelated to unlocking carrier lock.
Note that The AT&T and T-Mobile IPhone 7 variants, only come with a GSM chip. That means you can’t use an AT&T or T-Mobile iPhone on Verizon or Sprint, since those versions don’t have CDMA chips. (You can, however, take an AT&T iPhone to T-Mobile, or vice versa).
Also note, most of the entire world uses GSM, so you should be fine in 1st world countries like Europe. I'm not sure if other countries are dominated by GSM/CDMA but it's worth researching before travelling. Eg: Most of Northern Canada is dominated by CDMA coverage only, but this is changing as well (albeit slowly).
There are many options on where to purchase an IMEI unlock code, here is what I found after a quick Google.
The #1 IMEI provider I found for iPhone 7 and 7 Plus was OfficialiPhoneUnlock
PS: Shoot your sister some cash!
How I have unlocked phones in the past (Only tested on BlackBerry and Android):
Insert foreign SIM >> If it works, phone is already unlocked!
If it asks for IMEI unlocke code you have a few options:
- Purchase one online (I've paid as little as 1$ on Ebay for BlackBerry Code, to $30 for Android Code).
- Call Carrier and see if they will provide the code (It may be best to have original purchaser call, or at least be available to validate) (In Canada, this has became a legal right of the consumer on day 1 to ask for the code, possibly Europe has similar laws, I don't think the US does, but state laws may vary)
Enter IMEI Code >> Phone should connect and work immediatley (Try reboot if not)
If it doesn't then what you are looking for is called
APN Settings. Usually your phone will set this up for you, but if needed, you can use google or just call carrier. They should be able to provide this information even without an account.