at&t

If you mean calling it from a UK mobile while abroad... you would pay whatever your network's roaming charges are. Most UK based mobile operators have a list of countries where roaming is now the same as in the UK (FREE if you are uk resident). You will also have to check that 03 numbers are within your inclusive tariff

One will find that the latest network 3 mobile phones are not offered in the Americas, like North America (Canada or the United States of America). However, they can be purchased in the United Kingdom at a mobile network known as "Three" or "3".

There are various kinds of mobile networks based on the supported standards and technology. Some of these mobile networks are 3G and 4G. There are many operators like Verizon, Rogers, AT&T and Telus that provide mobile network solutions.

mBlox is the world's largest mobile transaction network specializing in providing operator connectivity and mobile billing capabilities to businesses around the globe. They are the intermediary between businesses and mobile operators managing the delivery and billing of mobile content and mobile services.

http://www.mblox.com/about/