A user uses a wireless communication device to engage in a communication session over a packet-switched network. The communication session is one of a plurality of different session types, e.g., a push-to-talk (PTT) session or a Web browsing session. The user is identified in the communication session by a user identifier that is dependent on the session type. The user identifier used in a particular communication session is included in a usage record generated for that communications session. A billing system determines the session type from the user identifier in the usage record. Based on this session type, the billing system selects a billing formula to calculate a charge to the user for the communication session.