Women Who Code iOS Study group Michele Titolo showed us how to check only for numbers in a UITextFIeld thanks for sponsoring us, SocialChorus!

ViewController.m

#pragma mark -
#pragma mark - UITextFieldDelegate
-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    // let's check if the string is range is a number
    NSRange characterRange = [string rangeOfCharacterFromSet:[NSCharacterSet letterCharacterSet]];
    
    // if we've detected a character in our string, we're not going to insert that letter character in our text field
    if(characterRange.location != NSNotFound) {
        return NO;
    }
    
    return YES;
}