How do you write a U.S. number in international format?

US mobile number format from UK

what is my mobile phone number in international format?

Do you mean what is the country prefix? I hope every country actually uses numbers as in 0u20139.

Assuming they do and that you have the telephone number of the person you want to call, all you should need at that point is a country code.

This (Country Codes, Phone Codes, Dialing Codes, Telephone Codes, ISO Country Codes) site here gives you all the country codes that you should need to dial anywhere you like.

Just enter the country code and then the number.