Carrier select

    //already done: unit test
    // suggestions of test case:
    //(1)set cac as 123, use a number begins with 123, 111
    //(2)set cac as 1234, use a number begins with 1234,1233
    //(3)set cac as 12345, use a number begins with 12345,12344
    //(4)set cac as 123456, use a number
    //(5)set cac as 12, use a number