So, for the record here's what my experience was:
My local Verizon store would/could not activate a new line with My TMobile iPhone 6 (and my wife's). Both phones are fully unlocked and support TMo, AT&T, and Vzn bands 3G/4G/LTE bands, but their activation process requires a Whitelisted IMEI to start service. Only official Verizon iPhones are whitelisted. Unlocked AT&T and TMo iPhones, while supporting all required bands, arent eligible. And depending on the level of experience the rep has, they either think "it wont work" (wrong) on their network or, that "it'll work, but we cant activate a line/SIM with it" (correct).
So, I actually got cheap ($22) VZN Droid IVs off of ebay and activated service. But those use micro-SIMs, not nano-SIMs. I got my service activated at one store saying "my wife and I want to try the service first before we commit with porting our numbers and our main phones" to Vzn. Of course, I'd have to pay, even if we decided to cancel.
The next day, i went to another store and said, we like the service and want to switch our main phones and numbers to VZN. Since they (or we) couldnt simply take the SIMs out and use them in the iPhones, they had to activate new nano-SIMs. Unfortunately, the same whitelisting issue came up even when trying to activate a new SIM on an existing line. So, it is a block on activating the SIM, not just a service line.
Now, I had to push them harder and come clean as to what I was trying to do. After a bit of explaining, and negotiating, I had them actually activate the new nano-SIMs on my account in a store display iPhone, then just give me the SIM and pop it into my phone. Did it for each mine and my wife's. This makes total sense. and I know others have done this. But it all depends on the willingness and savvy-ness of the rep/mgr you work with. So your mileage may vary. Just know that this process does work and will be supported by some of their people.
We've had the new service for 2 weeks since and no issues whatsoever. The VZN service in our area has greatly improved int he year since we left them and their pricing is much better now than it was. (Actually, we even got the bonus promotion that doubles our data allotment for the first year from 10Gb to 20Gb). And my company has a 21% corp discount. So for $110 for 2 lines one excellent network, i'm happy with this.