The cleanest solution I'm using in my apps is:
NSMutableCharacterSet *phoneNubmerCharacterSet = [NSMutableCharacterSet characterSetWithCharactersInString:@"+"];
[phoneNubmerCharacterSet formUnionWithCharacterSet:[NSCharacterSet decimalDigitCharacterSet]];
NSString* phoneString = [[phoneString componentsSeparatedByCharactersInSet:[phoneNubmerCharacterSet invertedSet]] componentsJoinedByString:@""];
No "if" logic, keeps the + in number, removes all kind of random unwanted characters
