AT$H12

N0    When originating or answering, handshake ONLY at the modulation specified by S37  

      and B commands.                                                                    

N1    When originating or answering, begin handshake at the modulation specified by S37 

      and B commands and allow fallback to lower speed.                                  

N2    If V.23 available, then include this modulation when fallback occurs in Answer    

      mode.                                                                              

N3    Originating, handshake only at the modulation selected by S37 and B. Answering,   

      negotiate starting at the highest speed selected by S37.                           

N4    Originating, handshake only at the modulation selected by S37 and B. Answering,   

      negotiate starting with the highest value set by S37 and include V.23 in sequence  

      if available.                                                                      

N5    Originating, begin handshake at the modulation specified by S37 and B commands,   

      fallback is allowed. Answering, handshake only at the modulation selected by S37   

      and B.