trying to reply to an email within Z431 ATT phone

Hello

I have a new Z431 cell phone from ATT, prepaid phone. It has email and I cannot get the instructions to reply to an email I received. When I press Rely, it takes my cursor into the body of the existing message but it will not let me type. It beeps at me. I do not want to have to make a whole new email just to reply to someone. Any ideas where I go to find instructions. On the ATT website it doesn't show me how to reply. Or I can't find it!. Thank you for your help in advance.