I purchased my 6T direct from OP. So it is the international version (dual sim).
My experience is plug and play (with caveat depending on what type of SIM you have). Sim 1: Verizon, Sim 2: Tmobile. Worked right out of the bat.
Now.. here is a requirement for Verizon that I personally found out. Check your SIM, it needs to say Verizon 4G LTE. I had 2 other iterations, V (from iPhone), Verizon 4G (samsung).
Transferring number to a different SIM, if you have extra one, is easy. I managed to change it online. At least for me when I changed SIM, I had to go through 'changing' my plan because the 4g lte SIM has 'different' features than my 'other' sim which to made little sense.
If you use the 'other' 2 sim I mentioned, your data and phone calls may work, but not sms and mms.
Again, I was able to do all this without calling Verizon. I have now verified and working LTE data transfers on Verizon, sms, mms, calls of course. Same with Sim 2: Tmobile, works just the same.
So yes, I am very happy with the 6T. I have waited for a very long time and spent too much money trying a dual sim for that will work with Verizon.
Hope this helps.