An arrangement comprising: first network apparatus of a first communication provider, configured to provide a first communication system accessible via a client application and packet-based network, and to maintain a first list of contacts being users of the first communication system; second network apparatus of a second, partner communication provider, configured to provide a second communication system accessible via a web-browser and the packet-based network, and to maintain a second group of contacts being users of the second communication system; and a user terminal installed with a web-browser for accessing the second communication system, and further installed with an independently executable client application for accessing the first communication system; wherein the client application is configured to establish a contact-sharing channel with the second network apparatus in order to fetch contact information of one or more of the contacts of the second group, display at least part of the fetched contact information, and establish a voice or video call with at least one of the contacts of the second group via the first communication system based on at least part of the fetched contact information, the call being established directly based on a user selection of the displayed contact information; wherein the first communication system requires users to be accepted as contacts to communicate with one another, and the second communication system requires users to be accepted as contacts to communicate with one another, and the client application is configured to establish said call via the first communication system based on acceptance of the at least one contact of the second group as a contact within the second communication system, without requiring acceptance of that contact as a contact within the first communication system. first network apparatus of a first communication provider, configured to provide a first communication system accessible via a client application and packet-based network, and to maintain a first list of contacts being users of the first communication system; second network apparatus of a second, partner communication provider, configured to provide a second communication system accessible via a web-browser and the packet-based network, and to maintain a second group of contacts being users of the second communication system; and a user terminal installed with a web-browser for accessing the second communication system, and further installed with an independently executable client application for accessing the first communication system; wherein the client application is configured to establish a contact-sharing channel with the second network apparatus in order to fetch contact information of one or more of the contacts of the second group, display at least part of the fetched contact information, and establish a voice or video call with at least one of the contacts of the second group via the first communication system based on at least part of the fetched contact information, the call being established directly based on a user selection of the displayed contact information; wherein the first communication system requires users to be accepted as contacts to communicate with one another, and the second communication system requires users to be accepted as contacts to communicate with one another, and the client application is configured to establish said call via the first communication system based on acceptance of the at least one contact of the second group as a contact within the second communication system, without requiring acceptance of that contact as a contact within the first communication system.