Question around Unlocking with TMO

For those of you who have either paid off your phones or bought them outright, how does TMO handle unlocking the phones? Do you lose any features or functionality like wifi calling etc? Been with TMO for about 3 years now and all of my phones have been unlocked( Nexus 4, Nexus 5, One Plus One, Moto X Pure).

I am paying for the phone up front and I just want to make sure that I understand all the factors with unlocking a carrier branded phone.

Thanks

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