What happens if I Unlock a Contract T-Mobile iPhone 4?

Okay so i have my sisters iPhone, she stopped paying her bills about a year ago so it's locked, meaning i can't phone anyone, buy apps, stuff like that. But what happens if I unlock it? Will i be able to call people, use a new t mobile chip and buy apps?

Bare in mind that's it a t mobile iPhone 4.

Only the provider can unlock it.