X4       X0 = blind dialing (does not listen for dial tone)
          X2 = listens for dial tone and tells the computer the
          baud rate with CONNECT 1200 CONNECT 2400 etc.
          X4 = like X2 plus BUSY and NO DIAL TONE.
          Be very careful on this one.  The highest X number is
          not necessarily the best.  The meanings of the various
          X numbers are VERY nonstandard.