Abstract: A method and apparatus for matching operation modes of modems for connection of terminals to a telephone line, i.e., matching operation forms determined by a communication speed, a coding format, a synchronizing format, etc., on both the calling and answering sides of the telephone line. When the answering side detects an extension specifying signal sent from the calling side, an operation mode of a modem corresponding to the extension specifying signal is used to select the specified operation mode from among a plurality of operation modes for modem circuits equipped on the answering side, for matching the operation mode of the answering side modem with that on the calling side. The answering side modem has a function of detecting a push-button dial signal serving as the extension specifying signal, and a function of providing the plurality of operation modes for the modem and optionally changing the operation mode upon an instruction from an answering side terminal.

Abstract: The operation modes of a plurality of modems in a communication system are matched with each other for connection of digital terminals to an analog telephone line on both the calling and answering sides of the analog telephone line. For example, operation modes of modems on both sides of the analog telephone line can automatically be matched with each other by transferring a mode instructing signal with DTMF, using multi-frequency DTMF symbols for a select signal employed in the telephone line, between digital terminals connected to both ends of the telephone line.