Using a longer value will keep your modem from reacting too quickly during a sudden noise burst. You can change the default setting easily. Programming the S10 register of your modem to a value of 30 will make the modem wait 3 seconds before disconnecting. This can even get you through a call-waiting bump. This is one of the most common problems and the most illusive to track down.
Most line noise is either sporadic or selective.
Sporadic line noise will happen whenever interference gets onto a phone line. Sporadic noise, by definition, happens only during some sessions and regardless of what exchange or area code you are calling. There is little you can do about the weather except call out on a sunny day. Radio and tv interference can come from oscillators or transmitters near your modem. Some modems are not properly shielded when manufactured.