How do you call a UK mobile in US?

The telephone country code for the United Kingdom is +44, and you must drop the trunk prefix 0 from the beginning of the UK domestic telephone number. For example, 07700 900123 becomes +44 7700 900123.

If your cell phone allows you to enter the plus symbol in a telephone number, the most reliable method is to enter the number in international format, beginning with the plus symbol. (The details of how to enter the plus symbol depend on the make and model of your cell phone.) Otherwise, substitute the US international access prefix 011 for the plus symbol.