See the mini-howto: Leased-Line which covers leased lines where there is no dialtone. Leased line modems are analog and not digital. These special modems are used on lines leased from the telephone company or sometimes on just a long direct wire hookup. They often will also work as ordinary modems but go into leased-line mode when the AT command &L1 is given.
Ordinary modems for a telephone line will not normally work on such a leased line. An ordinary telephone line has about 40-50 volts (known as the "battery) on it when not in use and the conventional modem uses this voltage for transmission. Furthermore, the telephone company has special signals indicating a ring, line busy, etc. Conventional modems expect and respond to these signals. Connecting two such modems by a long cable will not provide the telephone signals on the cable and thus the modems will not work.
Leased-line modems often use a "dumb mode" where they ignore AT commands, disable result reporting, etc. One type of leased line used two pairs of wires (one for each direction) using V.29 modulation at 9600 baud. Some brands of leased line modems are incompatible with other brands.
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