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If you have ever attempted to call an international number using your modem or FAX machine, you may have encountered problems getting the modem to work properly. The most common problem in getting calls initially established is the differences in the dial tone, ring-back and busy signals, which vary from one country to another.
Most of the common models of modems are not equipped to deal with the changes in the pitch and cadence of a busy or ring-back signal, causing these signals to be incorrectly detected.
Modems that have voice capabilities, such as those with answering machine, voice-mail, or speakerphone functions are usually the ones that have the most problems when calling telephone numbers outside the country that they were sold in.
A related area of concern is using a modem in a country other than one that it was sold in. Most modems do not work well on phone systems other than the one it was designed for, and some modems and laptop computers can be seriously damaged by connecting them to a phone system that they are not approved to be used on.
Some modems do have the ability to work with the telephone systems of more than one country, and these modems usally come with a set of special cables or adapters that allow the modem to be connected into the local telephone service jack. Even if the modem is legally and electrically capable of being connected safely to the telephone system in a given country, the modem may require setting changes to work properly when it is used in other countries.
For example, most brands of modems do not like the intermittent dial-tone that the telephone network in Italy provides. These modems will usually falsely report "NO DIAL TONE" since they do not detect a constant dial tone like the modem is expecting. Some voice-mail systems also cause the dial-tone to stutter, causing similar dialing failures, but only when new voice-mail messages are waiting to be heard, making reliable dialing unpredictable.
To deal with calling to another country or placing calls from a country other than the one where the modem was sold, there is a reliable solution. Nearly all modern modems are compatible with the EIA 602 standard, (known as the "Minimal AT Command Set"), and that standard includes the "X1" command. The X1 command instructs the modem to make no attempt to detect dial-tone before dialing a number. Instead, the modem goes off-hook and waits for the number of seconds specified by the S6 register and then begins to dial the numbers, assuming that dial tone should be present. Most modems use a default value of 2 for two seconds. If it takes longer than two seconds for dial-tone to usually appear after going off-hook, increase the setting of the S6 register (S6=n) prior to dialing.
The X1 command also instructs the modem to make no attempt to interpret the ring-back or busy signals returned after all digits are dialed. If the modem wasn't going to correctly recognize these signals anyway, ignoring them completely is best. The modem will listen only for a modem carrier. The modem will wait for carrier for a period of time specified by the S7 register. If carrier isn't established within the number of seconds specified by the S7 register, the call aborts, the modem goes on-hook and will report "NO CARRIER". The V.90 protocol requires at least 35 seconds to train and become connected, plus the amount of time it took for the distant modem to be reached and for that modem to go off-hook. Subsequently, a S7 setting of 60 seconds is reasonable in most cases, but a longer amount of time may be required for overseas calls. (Lower speed modem transmission protocols train and connect faster, so they do not need as much time as V.90 does.)
The Xn command has several combinations available, so it is possible to ignore only the dial-tone (known as "Blind dialing"), ignore only the ring-back and busy signal, ignore both or ignore neither. A general command reference is available on this site, but the manual that came with your modem will have the precise commands that your modem will accept and should also state which countries it is approved for use in.
This information is provided on an "AS IS" basis and contains no warranty. Most countries have regulatory authorities who will have the latest specifications and compliance information.
International Telephone Busy Signal Reference (HTML)
International Telephone Ring-Back Signaling Reference (HTML)
Return to the Data/FAX Modem Reference Index (HTML)
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