What prefix does a UK mobile use to ring a US number while in the US?

The most reliable way to dial any telephone number on a GSM mobile, especially when roaming internationally, is to dial the number in full international format, beginning with the country code (including the plus sign).

The country code for the United States (and Canada and certain nearby islands) is +1.

When roaming in the US, you may be able to dial US numbers with just the area code and number; for example, 816-555-0123. However, if this method fails, use the full international format +1 816 555 0123, including the plus sign.