A charging trigger function (CTF 40) in a communication network (30) advantageously uses pre-allocated usage units to authorize the use of a communication service by a user, when a pre-allocated usage unit is available with respect to the involved communication service and user. A pre-allocation function (PAF 42) provides the one or more pre-allocated usage units directly to the CTF (40) or indirectly via an online charging function (OCF 44). The pre-allocated usage units are determined, for example, according to predicted usage information, user account information, and configured rules. A usage prediction function (UPF 52) in the network (30) uses user-related information, such as historical usage data or other activity by the user, to generate the predicted usage information. Among its other advantages, authorizing the use of the communication service based on pre-allocated usage units obviates or reduces the need for usage authorization signaling going between the CTF and OCF.