Couldn't he use the 1520 on T-Mobile and maintain Wi Fi calling if the 1520 is unlocked?
No, unlocking doesn't change that its not loaded with a T-Mobile ROM. T-Mobile Wi-Fi calling requires a special app. It won't be offered or installed on other phones. If T-Mobile offered a 1520 that was hardware identical, in theory you could load the T-Mobile ROM and be good to go but T-Mobile never sold any 1520s.
I don't know why the T-Mobile Wi-Fi calling app isn't offered for BYOD devices. I hope its because it couldn't possibly work without special firmware. If its because they just want to pressure you to buy phones from them, well that's very un-un-carrier like. So I'll believe its a firmware issue.