Embodiments of the present invention include controlling user subscriptions within an IMS Application Server of an IP Multimedia Subsystem, IMS, network, via an Open Service Access Application Programming Interface, API, between the IMS network and a web-based application. Embodiments include subscribing a set of users by sending a start subscription message from said web-based application to the IMS network, the start subscription message including an address for each user in the set. An acknowledgement of the subscription is sent from the IMS network to said web-based application, the acknowledgement including a correlator uniquely identifying the subscribed set of users within the IMS network. A subscription for one or more users of the set is terminated by sending a stop subscription message from said web-based application to the IMS network, the stop subscription message including the address of the or each user for which the subscription is being stopped.