not true if you travel much you definitely want t mobile for wifi calling... I did about 15 weeks on the road last year...most of that international but as long as there was internet access via wifi I always had full cell coverage as if I was still at home at the same cost as if I was home
To add on extra thing on using wifi coverage.
I was in Tokyo last year ..one of the guys rented one of thoese little portable internet hot spots from the local cell company...he let me use his wifi signal.. so my US T Mobile phone was connected via this wifi and i was making calls and texting to the us at US local rates.
What this mean is if you travel overseas you can just get a local phone with hotspot and use it for local incountry calling and use the hotspot to connect you US Tmo phone for US calling at US rates.
Ive even done this in the US...had a Verzion guy hotspot his phone and let me call with my TMo phone over his connection.. crazy