Unit testing a class method in XCTestCase

I am currently beginning writing some unit test in my project. However, seems that I met some issue:

- (void)testThatItIsValidNumber
{
    BOOL result1 = [NSString isValidNumber:@"1234567890987654"];
    BOOL result2 = [NSString isValidNumber:@" ^123dadaj"];
    BOOL result3 = [NSString isValidNumber:@"123 09 123"];
    BOOL result4 = [NSString isValidNumber:@"~#)(^%@*#&Bhadahj1@"];

    XCTAssertFalse(result1);
    XCTAssert(result2);
    XCTAssertFalse(result3);
    XCTAssert(result4);
}

This gave me an error:

caught "NSInvalidArgumentException", "+[NSString isValidNumber]: unrecognized selector sent to class 0x1069448e0"

Any one can guide me to the right way?

Edit:

+ (BOOL)isValidNumber:(NSString *)value {
    const char *cvalue = [value UTF8String];
    unsigned long len = strlen(cvalue);
    for (int i = 0; i < len; i++) {
        if(!isNumber(cvalue[i])){
            return FALSE;
        }
    }
    return TRUE;
}