A web application assigns a number that resembles a conventional telephone number and that uniquely identifies the current Web session, and injects this number into a signalling message or signalling messages that flow towards a WebRTC gateway. The WebRTC gateway inserts this number into the signalling information element that conveys Caller ID in the signalling message that it sends into the telephone network. This allows the recipient of a WebRTC call to identify the web session from which the WebRTC call originates.