In telephony, call control refers to the software within a telephone switch that supplies its central function.
Call control software, because of its central place in the operation of the telephone network, is marked by both complexity and reliability.
Call control systems will typically require many thousands of person years in development.
The required functionality and reliability of call control is a major challenge for Voice over IP (VoIP) systems.
VoIP systems are based on Internet standards and technology which have not previously attempted to satisfy such complex and demanding requirements as those that specify call control.