You'll need to get your phone unlocked before leaving. Not sure how you do this, but I'm sure Verizon could direct you.
I'd recommend getting a contract with Three when you are here. Their contracts have a "Feel at Home" option which means when you travel (about 40 countries are covered), you can use your phone, just as if you were in the UK. And the US is one of the countries. So when I go home, I can call my husband as much as I want, use WHATSAPP with my friends, use the maps for navigation, use all the data I want for no additional fee.
I just use Facetime and WHATSAPP, so I don't have a US number. I'm sure someone else will know more about that stuff than me.