In the paper we present the software reliability and maintenance concept, which we use in the software development, testing, and maintenance process, for automatic call distributor MEDIO ACD. The concept has been successfully applied on systems, which are installed and operational in Moscow and Saint Petersburg, Russia. In the paper, we have concentrated on two main issues, as follows: (i) Set of fault-tolerant mechanisms needed for the system exploitation (error logging, checkpoint-restart, overload protection and tandem configuration support). (ii) MEDIO ACD software maintenance concept, in which quality of the new software update is predicted on the base of current update's metrics and quality, and new update's metrics. This forecast aids software maintenance efficiency, and costs reduction.