Hopefully they didn't take out the VoLTE feature in the D6603 versus the T-Mobile version the D6616 which is the only US version since the Verizon Z3v is really only a Z2.5 lol
It does take some time to go from the GS5 to the Z3 especially if you are like me and I go back and forth between them although over the last month I have had it I probably have used the Z3 about 75% of the time some of that to force myself to learn it. The Noise Cancelling ear bud headset (Sony MDR-NC31EM) work awesome with it. I never thought I would say this but I miss somethings from TouchWiz but it's hard when the Z3 just feels so much better and quality as a phone with the exception of the back being glass it slides way to easy.