recieving calls from UK on cell phone

I got a call from my girlfriend who is studying in the UK yesterday. She just got her cell phone there and didn't know much about it, and also didn't know her phone number. When she called me, on my verizon cell, the number came up like this

816-xxx-xxxxxx (thats 3 numbers, then 3, then 6, for a total of 12)

My question is, when I try to call this number back, it says the number is incorrect and to check the number and try it again. When I looked up UK phone numbers online, none of them seemed to have 12 numbers. I'm calling using a calling card that it acceptable in the UK. It's not necessarily a matter of calling from my verizon phone, although I am calling from it with the calling card. Does anyone know how to determine a UK phone number when it comes up on your cell phone? Thanks for the advice.

hrm, well how does she call you? I would imagine you have to dial the country code first.