No dial tone in phone line -- make sure that one end
of your phone line is connected properly to the telephone wall socket while
the another one is firmly plugged into the line socket of the modem.
Digital phone line -- most offices are using Digital PABX phone
system. If you get no dial tone while using the office phone line for the
first time, it is probably a digital phone line. Try using the fax line. If
this work, you may need to get separate phone line.
Unusual dial tones -- if you have applied for some additional phone
services such as call transfer, there will be some unusual dial tones that
last as long as several seconds. Your modem always listen to the phone line
and will conclude that there is no dial tones or busy tones.