To you it may sound like a dialtone but to the modem it does not. The pitch may either be not loud enough or too loud. Try giving the ATX0 command before you dial. You may also want to look at your phone cord; is it "straight through" or are the wires crossed? You may also want to try unplugging some other equipment plugged into the modem or into the same line.
The modem does not recognize the ringing signal as a ring. This happens when dialing from a PBX or Keys System, or dialing long distance or international. Try giving the ATX0 command. Also, check to make sure your phone cord does not have wires crossed, and make sure there are not many other things plugged into your line like answering machines.
Usually, this is caused by poor line conditions. Try calling again, or calling a different number. If you are calling long distance, try using a line without Echo Cancellation; your long distance carrier should be able to tell you what prefix to dial to do this. This may also point to interference of other equipment on your line. For example Alarm systems, Fax machines, answering machines and other things that use modems (Such as Credit Card machines!) are suspect. Try unplugging these.
Another cause can be an incompatibility between your modem and that at the other end; try turning of Data Compression, dial again. If that does not work try turning off error correction, and try again. If that does not work, try forcing your modem to dial at a lower speed (For example, 9600 instead of 14400).
You probably have a noisy line from you to the other modem. This problem can literally be anywhere. Make sure you have a healthy phone cord, make sure there is not too much equipment on your phone line such as answering machines, faxes etc. etc. Try dialing in at a lower speed, to a different number, too. Try what happens when you turn of Retrain, Fallback or Data Compression / Error Correction.
You lost the other side. Look at the S switches that set the time you will try to retrain, make sure there is no one picking up the phone on the line you are using. Make sure call waiting is disabled; make sure there are no devices such as Fax Machines or Answering Machines or Alarm Systems trying to use the line.
Congratulations, you are the proud owner of a dirty line! You can check your inside wiring, to make sure it is correct, and all your connections to the line. Also check that your phone connection, if outside, is absolutely dry. The Phone Company can usually come out and attempt to fix this (mostly without success) for a hefty fee. You should notice your line also hisses when it rains.
Well... This is not an Exact Science! Numerous conditions can cause this, including the way calls are routed in the switch, the traffic on other lines, etc. etc. Try to find a pattern in this problem.
Check your Flow Control; make SURE both Computer and Modem are set for Hardware Flow Control, and Hardware Flow Control ONLY!
Make sure your software uses RTS/CTS Flow Control, and NOT DSR/DTR flow control! What will happen when your buffer is full is that your computer will drop DTR to signal the modem to stop sending. But the modem reads this as: Hang up the line, which it promptly does!
Check to make 100% sure you have your software set for 8 Databits, No Parity, and One Stop Bit. Usually, this problem is caused by 7 databits, even parity, One Stop bit setting.
You probably had a buffer overflow. Here are a few things to try: Make sure you use hardware Flow Control, and Hardware Flow Control ONLY! Make sure you use a correct serial port speed that is not too high for your port. Try 38400 or lower (19200, 9600 etc.) Make sure the modem is setup for hardware flow control.
Make sure you have a full cable that holds all 8 active pins! Radio Shack sells Gizmo's you can plug in your modem to verify all lines are working. If you have an internal modem, you're out of luck.
Probably, you lost Sync on the Synchronous part of your connection (The DCE-DCE part) and error correction is not yet aware of this, or unable to correct it. Hang up and dial again. If this keeps on happening, turn of error correction and/or compression.
Make sure you are set for 8 databits, No Parity, One Stop bit. Make sure you use one of the supported bps rates (38400, 19200, 9600, 2400, 1200, 300).
Probably your modem is not properly set for Interspeeding or Speed Adoption. Try dialing at 9600 and see what happens. Especially if you have a slow (say.. 2400 bps modems) try this! In general, modems will not talk faster than 4 times their top speed in Modem mode. Modems without compression often do not talk faster than their top speed or the next notch over it (i.e. 19200 for a 14400 modem). Some modems that are 14400 will not talk faster or slower than 14400. This is not a speed the UART deals with very well; you will probably run into a lot of trouble.
This technique will allow for every time unit on the line to carry more than just one piece of data, and that is where the difference between Baud and bps comes in. The Baud rate on a line indicates how many changes in tones per second there are; for example, on a 300 baud line there are 300 changes per second. On a 2400 baud line there are 2400.
The bits per Second rate explain how many bits can go over the line. For example, on a 300 bps modem connection, there is a 300 baud link, so there are only two sets of frequencies in use. On a 2400 bps line, there's 2400 baud speed, and one bit per baud unit