At least one MULAP line in a communication installation is controlled and/or monitored using an application. To simplify the combination of MULAP lines and CTI applications, or to allow it at all, each MULAP line has an associated MULAP call number, and a plurality of terminals can access these MULAP lines, each of the MULAP lines having a respective associated terminal as master terminal. First, the application sends an order for a MULAP call number to the communication installation. The communication installation then converts the MULAP call number into the terminal call number of the master terminal associated with this MULAP line and carries out the order for this terminal call number, all messages relating to MULAP line being sent as those associated with the master terminal from the communication installation to the application as messages associated with this MULAP call number.