If you are experiencing lag, either with an analog ("56k") modem or with
DSL, pick up a phone on the same phone line and dial part of a number (e.g.
press "5") to get the dial tone to stop.
You should hear nothing. If you hear crackling or static, this is probably
the source of your Internet connection problems. Both analog modems and DSL
can't deal with static on the line, and will occasionally either lag for
several seconds or, with analog modems, disconnect.
You can get the phone company to come out and look at it by dialing 611 (in
most of the US) and complaining about static when you are talking on the
phone. You don't want to mention anything about modems, the Internet, or
DSL or they'll get really confused and either want to sell you more services
or walk you through your computer's Internet settings. The important thing
is that they fix the static that you hear when you talk on the phone; if
they do that, your internet connection will be more stable. They will
likely be willing to do this for free.