What is "compatible". Really nothing if not exactly said
              with  WHAT.  "HAYES  compatible"  is  to  vaque.  "HAYES
              command set compatible" is better.  Most modems,  so not
              all, are only compatible with a "subset of HAYES command
              set".  But  also  most "hayes" modems have commands  not
              existing  with  hayes.  Some  unlucky  vendors  did  use
              commands  or registers not used with hayes-1200 at  that
              time, but later used in the HAYES-2400. (Bad Ques).
              The best I can say most of the better clones are more or
              less compatible upto HAYES-1200.
              As  I  don't know if besides HAYES-1200 and  the  HAYES-
              2400,  also a HAYES-300 exists,  I further only refer to
              this two,  from which both I have the specifications. In
              Europe a lot of 'hayes' modems with only the V21 and V23
              speeds  exists,  which  are similar,  but  absolute  not
              connectable, with the BELL-103 and BELL-202 norm modems.
              As most application programs only use a small set of the
              HAYES-command  set,  most  clones will  run  with  these
              packages  such that the user is pleased,  not to use his
              fingers and ear to make a connection.  This if he has  a
              user friendly telephone company (are there),  but mostly
              the  results are hopeless in peak  hours,  if there is a
              mismatch  between  the modem commands and  the  terminal
              commands.  I  had a modem and a terminal program and  on
              some day no nice CALL <my mother> was succesfully.  With
              finger  and ear there was no problem.  Reaction  of  the
              terminal was "BUSY".  The real point was,  the modem did
              not pickup in ATX2 mode the dial tone from the Telephone
              company,  so didn't start dialling. As the modem had not
              a  speaker  (for  cheapness), I could not use  the  ATM1
              command,  to  follow  the connection process.  The  dial
              signal was strong enough, but the frequency was a little
              ouside (WHO's) specification.