Make sure you do not have call-waiting on the phone line your modem uses . If you do, turn it off when dialing the Internet. Poor line conditions cause the vast majority of unexpected disconnects. If you can hear any noise on your line, then its very likely you will not be able to connect reliably. If you suspect line noise, give your phone company a call and ask to have your line checked for noise.
Some telcos provide a data-quality line for modem use.
In rare cases, modem incompatibility can cause random disconnects . We've seen this with some older generic modems. Replace the phone line from the back of your modem to the wall . Some low grade cable can cause problems. Try calling from a different location .