This will maintain the text field to be formatted like a phone number, and have a placeholder as the user types in the number, i.e. "(12 ) - ". And all while maintaining the cursor position where the last number is typed.

UITextField Phone Number

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
   // new number
   NSString *phoneNumber = [textField.text stringByReplacingCharactersInRange:range withString:string];
   // strip non-numbers
   NSString *rawPhoneNumber = [[phoneNumber componentsSeparatedByCharactersInSet:[[NSCharacterSet decimalDigitCharacterSet] invertedSet]] componentsJoinedByString:@""];
   
   if(string.length == 0 && rawPhoneNumber.length == 3){
      // delete again
      phoneNumber = [phoneNumber stringByReplacingCharactersInRange:NSMakeRange(range.location-2, range.length) withString:string];
      // strip non-numbers
      rawPhoneNumber = [[phoneNumber componentsSeparatedByCharactersInSet:[[NSCharacterSet decimalDigitCharacterSet] invertedSet]] componentsJoinedByString:@""];
   
   } else if(string.length == 0 && rawPhoneNumber.length == 6){
      // delete again
      phoneNumber = [phoneNumber stringByReplacingCharactersInRange:NSMakeRange(range.location-1, range.length) withString:string];
      // strip non-numbers
      rawPhoneNumber = [[phoneNumber componentsSeparatedByCharactersInSet:[[NSCharacterSet decimalDigitCharacterSet] invertedSet]] componentsJoinedByString:@""];
   }
   
   // pad and trim to 10 digits
   phoneNumber = [[rawPhoneNumber stringByAppendingString:@"          "] substringToIndex:10];
   
   // add in format
   phoneNumber = [NSString stringWithFormat:@"(%@) %@-%@",
                     [phoneNumber substringToIndex:3],
                     [phoneNumber substringWithRange:NSMakeRange(3, 3)],
                     [phoneNumber substringWithRange:NSMakeRange(6, 4)]];
   
   [textField setText:phoneNumber];
   
   NSInteger offset = rawPhoneNumber.length + 1;
   if(rawPhoneNumber.length >= 3){
      offset += 2;
   }
   if(rawPhoneNumber.length >= 6){
      offset++;
   }
   
   // place cursor where use left off
   UITextPosition *pos = [textField positionFromPosition:textField.beginningOfDocument
                                             inDirection:UITextLayoutDirectionRight
                                                  offset:offset];
   UITextRange *newRange = [textField textRangeFromPosition:pos toPosition:pos];
   
   //select it to move cursor
   textField.selectedTextRange = newRange;
   
   return NO;
}