The disclosure relates to a method of establishing a circuit switched, CS, call to a mobile subscriber of a CS network, wherein the mobile subscriber includes an instant communication client which is adapted to communicate via an instant messaging protocol with an instant communication server. The instant communication server is associated to a hosting switching node of the CS network. A switching node of the CS network receives a CS setup message for terminating a CS call. A switching node of the CS network detects that the mobile subscriber is not reachable over the CS network. The CS setup message is routed to the hosting switching node. A message converter associated to the hosting switching node converts the CS setup message into a converted setup message according to the instant messaging protocol. The instant communication server forwards the converted setup message to the instant communication client.