There are many reasons why you can lose your connection. Some you can fix, others even the telco may have difficulty with. Given a reasonable line and decent quality modem you should be able to enjoy a solid connection. 

When we talk about a disconnection we mean that the modems have dropped the line (or, if you prefer, hung up). We aren't talking about when a site doesn't respond or breaks off during a transfer. In general, these things are beyond our control. 

If you are asked to speak to your phone company make sure they understand that you are using the line for data transmission. Data transmission is more demanding than ordinary voice transmission, and it is more sensitive to problems with the line. What is fine for voice, may be absolute rubbish for data communications. The phone companies are still getting used to the idea of residential customers using modems. 

Drops Immediately After Login

This seems to be a problem with PC's more than with any other computer. You are able to login without problem, but then get no response or error messages from your program. It looks as though the COM port has been set to 7 bit data mode instead of 8 bit data mode required for TCP/IP. This is good enough to permit the login to happen, but then when it switches to the protocol, it fails. After a minute or so the line will be dropped. 

One solution is to turn everything off, modem and computer, restart the computer, go back into Windows of whatever variety. Then when it is ready, turn the modem on (if it is external) and try again. 

Call Waiting

If you have call waiting on the line you use for your modem then that can cause unwanted dropped lines. The modem is not tolerant of the call waiting beep. You could modify the login script to disable call waiting and modify the logout, or bye, script to re-enable it. 

Inactivity Timeouts

The TCP/IP software and Internet Browsers may include an inactivity or idle timeout. If this is set to anything then it may drop the line on you while you are reading a web page, for example, or your mail. 

Leads and Adapters

Make sure the phone cable, any extension leads and adapters, that connect the modem to the telephone socket are not damaged in any way. Try and plug the modem directly into the telephone socket if only to rule out the possibility that something could be faulty with them. 

Ensure the cable is securely plugged into the modem. The flimsy clip that keeps the cable in the modem can easily break off and if it is it doesn't take much for the cable to dislodge slightly causing a disconnection. Loose or damaged modem cables are the main cause of NO DIALTONE error messages. 

Faulty Modem

Your modem may be at fault. It is possible for it to work but to be on the border line. Borrow one from a friend and see if it makes a difference. When buying a modem, don't forget that you get what you pay for. Ask around, discover other peoples experiences. Don't be seduced by the latest all singing, all dancing gizmo. It may be staid and not very pretty but it may well do a better job. 

Line Quality

The quality of the line may be poor. Plug in a phone instead of the modem, dial someone and listen to the phone line. Are there any crackles and/or any other noises? If so, complain to your telephone company. 

Your ISP

It is possible, just about, for the problem to lie with us. Maybe one of our modems or associated equipment has given up. And if your ISP was too busy you would get a true busy signal.