A method may comprise receiving, from a first handset via the Internet, a subscription message for subscribing the first handset to a service for receiving packet-switched messages other than SMS messages, EMS messages and MMS messages. The method may further comprise storing information indicating that a first mobile phone number of the first handset corresponds to a subscriber of the service, in a subscriber data store. A first request may be received, from a second handset, the first request including at least information corresponding to the first mobile phone number of the first handset. The first mobile phone number of the first handset may be determined to correspond to a subscriber of the service. A first response may be transmitted to the second handset.