A method for handing over a call from a first communication network to a second communication network, comprising the steps of: registering, by a first mobile client, in particular, an IP-PBX VoIP client, running on a mobile device, the mobile device with an IP-based PBX for the first communication network, following a naming convention, by sending a registration message from the first mobile client to the IP-based PBX, the registration message comprising an endpoint ID, establishing an active conversation over the first communication network, detecting, on the mobile device, a connectivity change with respect to the first communication network, wherein the further steps of retrieving, by the first mobile client, the status of the call impacted by the connectivity change, and subsequently transferring the call to the second communication network are carried out using SIP NOTIFY messages. Further, a communication system for carrying out the method is provided.