A communication system which includes two digital terminal stations connected to each other by a digital communication line. The communication system includes data terminals adapted for operation on an analog communication line which transmits main data and control data. The data terminal has two modems for transmitting respective data. Correspondingly, the digital terminal station has two modems which modulate and demodulate the respective data signal for transmitting the data from the digital communication line through the analog communication line and vice versa. The data transmission between two data terminals through the digital terminal stations is completed in one transmission call.