It depends on what mobile system do you have on your phone.

If you are android user visit google play store.

In mobile communication, the interface refers to a program that controls the display for the user thereby allowing the user to interact with the system. It also refers to how various users connect with a number of mobile platforms.

what are the objective of operating system

To call a mobile phone when the user is roaming internationally, dial the mobile number exactly the same way you would when the mobile user is at home.

The objective of a mobile phone company is the same as any other company, money. The phone companies do this by producing sales.