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Solutions (Try these first)
1. Install a surge suppressor on your phone line between the wall jack and the computer.
Explanations(Read on if necessary)
Noisy telephone lines from you to your telco provider:
This is common among neighborhoods more than 5 years old.
Faulty telephone equipment inside your home:
If you have telephone equipment that is hooked up to the same telephone number as the line your computer modem is on, it is possible for this equipment to inject data/noise into the phone line. Bear in mind that this type of failure is often not detectable to the human ear.
You can get a crystal clear voice phone call at less than 2400 baud, this means you will not hear any noise that exists, and be able to connect at 2400 baud. You, however are connecting at more than 10 times that speed when you connect at only 28,800 baud (maybe 20 times if you have a 56K modem), and modems detects every bit of data that is out of place, even and especially in-audible errors. Data streams have a checksum bit, this means your modem gets a chunk of data, performs a little math on it and comes up with a number, if this number matches the accompanying checksum, the data packet passes and the next one is requested. If the test should fail a re-transmission is requested, the occurrence of the re-transmissions is tracked by the calling modem and if there are too many the modem will disconnect the line, giving the call up as not worthwhile. (I personally have had 2 instances of this; one was an old cordless phone emitting noise that was not even detectable to the human ear, into the phone line, even when not in use. The other was an answering machine that was about a week old. I found these problems through simple deduction. I unplugged everything from the wall jacks while I was using the computer, this made a noticeable difference, so one by one I plugged items and phones back in and used the modem to make a call again until I found the problem equipment, the new equipment was warranty replaced. Also this answering machine also worked notably fine, crystal clear voice messages were saved and played and it had no glitches except that it was interfering with my modem internet call by some means undetectable to the human ear. Since those 2 occasions, I have used a dedicated phone number for computer use.)
Another possible solution is a modem INITITIALIZATION ("init") string:
Frequent disconnects can be caused when a modem has to make to many retransmissions and/or retrains to maintain line integrity and it just gives up and terminates the call. The proper INIT string can fix this. One item that leads me to believe it may be noisy phone lines is that your computer locks up when this happens, a modem data call should never cause your computer to lock up. Line voltage surges can plague computer modem connections. NEVER run a line from the wall to the computer. ALWAYS place a surge suppressor between your computer modem and the wall jack. A telephone line has approx. 45 volts that doubles to 90 volts AC when it rings. Telephone lines are infamous for voltage surges. DO NOT depend on your modem to handle a voltage surge above 90VAC when it is plugged into a system that has a processor that runs at 3.5 VDC or less. This is what causes lockups, and voltage fluctuations.
We hope this has somewhat helped you to understand what occurs when a modem call is made.