Will an unlocked iphone 6 work on Verizon in the US?

If it wasn't a Verizon phone originally then it will not work right on Verizon's network, because Verizon officially only supports phones that are registered in its database of Verizon-specific phones. In other words, Verizon does not support Bring Your Own Device (BYOD).

Some functions will work if you insert a Verizon SIM that has already been activated using another Verizon phone, but only on Verizon's LTE network. If you lose LTE coverage it will not fail over to CDMA. Also, SMS (text) may not work, and Visual Voicemail may not work.

Finally, if you have a problem Verizon will not provide support to resolve it.

Theoretically, an unlocked iPhone 6 would work on Verizon's 4G LTE network if you have an already active Verizon SIM. However, the phone won't work if you are somewhere that Verizon only has 3G. Some features, such as Visual Voicemail may not work. Verizon won't provide support if the phone doesn't work. But, it also may just not work on Verizon's network.

If you buy an iPhone 6 from Verizon, it's unlocked. You can use it on Verizon and then later on AT&T or TMobile (but not Sprint).