Not to much to go on at the moment with what you discribe.Would help to know a little more on your system spec.
Would suggest that you check that your system is up to the mim requirements for running your isp.
Does your system have enough memory,hardrive space.Run scandisk,defrag and empty any temp files,including temp internet files.Are there to many programs running in the background ? Do a repair to internet explorer
Do a check on your modem via control panel,see if there are any updated drivers for modem.
Is your phone line set on call waiting? or is your phone noisey (check out your line)
Are your settings for modem in device manager correct? Any other number you can use to connect to your ISP.?
Try uninstall of ISP software then reinstall.
Generally, there will always be an occasional disconnect. if you're getting a lot more than the occasional disconnect, something is wrong and you should methodically follow the steps below.
Follow link for more info.