Your Brother machine's Manual Receive Mode completely disables automatic fax reception. You should only use Manual Mode if you do not receive many faxes.
Note
If the machine's Easy Receive function is on, then you do not have to press the Black (Mono) Start or Color (Colour) Start key on the Brother machine to receive a fax. Simply wait until you hear the machine's receiving tones, hang up the telephone's handset, and your machine should receive the incoming fax automatically.