To convert sph file to wav format with sph2pipe use
sph2pipe -f rif file.sph file.wav
To convert file with SOX use
sox file.sph file.wav
although complex compression methods like shorten might not be supported. For your particular file it will work fine:
HTK can handle such files perfectly too, for details see the section 5.11.4 NIST File Format in HTKBook. You need to configure SOURCEFORMAT=NIST in config file. For switchboard specifically you also need to split channels with STEREOMODE set to LEFT or RIGHT.