If you are dialing a country other than the US, whether you are calling from the USA, another European country, or from inside that country, and you are using your US mobile account, you start with the +, followed by the country code.
If you are dialing a country other than the US, whether you are calling from the USA, another European country, or from inside that country, and you are using your US mobile account, you start with the +, followed by the country code.