Installing software to get a few free hours on some online service can often result in this headache. This is one of the toughest problems to diagnose because there are so many potential causes. Any irregularity in the copper lines or switches or even external factors like nearby lightning can cause noise. Modems are designed to filter out noise but this is not foolproof with even the highest quality analog modems. Occasional dropped connections will happen.
Frequent disconnections are a sign of other trouble though.
Occasionally, HTC Technologies must replace, upgrade, or repair our equipment which can result in lost connections and services being unavailable. We do our best to announce these things ahead of time and to schedule them during our maintenance period. Equipment can fail without warning at any time. We try to post announcements when this happens as well. You may have set in your connection software a disconnect after 'x' minutes of inactivity.