This invention relates to a method and apparatus for charging and billing of Circuit Switch, Packet Switch, and IP Multimedia Subsystems in third generation telecommunication systems. The charges for a call are computed by a rating engine, a software system comprising data tables and a program for calculating charges from the parameters of the call, such as the calling and called numbers, the length of the call, and special charge items such as priority service. For calls whose billing cannot be deferred, referred to as on-line billing calls, the rating engine is accessed immediately. For calls whose billing can be deferred, referred to as off-line billing calls, the rating engine performs its function only when the rating engine is not busy rating on-line billing calls, for example, during periods outside the busy hour. Advantageously, only a single rating engine is required, and the capacity of that single rating engine is determined only by the busy-hour volume of on-line billing calls.