I believe Verizon was required as part of its win of the 700 MHz auction to open its phones using this frequency.

In the case of T-Mobile, they have published rules about unlocking devices. In fact, there's an app on my wife's new S8 phone that enables it be unlocked, so there's no longer a need to call to get the unlock code. Any active phone can be unlocked after 40 days as long as there is no Equipment Installment Plan on the line.

However, with T-Mobile's included international data, I don't find myself searching for a local SIM when I travel.