Method and apparatuses for collecting user activity information in a telecommunications system. A first telecommunications node (120) handling signaling of the terminal of a user sends a message (516) notifying a registration event (515) of the user to a second node (130) handling registration information of said user. The message comprises an information element (ACT) indicating whether the user has been involved in a service provided during a registration period comprised between a previous registration event (501) of said user and said notified registration event. At reception of the message, the second node updates registration information (430) and activity information (440) of the user. By including service usage information in registration notification messages, the operator of the telecommunications system can obtain information about the rate of service usage in the system without requiring implementation of neither new nodes nor new messages to be exchanged by new or existing nodes.