A method of acquiring interoperability between a push to talk server and a chat server is disclosed. A push to talk server comprises an interface adapted to receive requests from another server to send messages to push to talk clients via the push to talk server. A chat server comprises an interface adapted to send to the push to talk server a request for permission to send chat messages to push to talk clients. A computer program product is described for communicating messages in a communication network comprising a push to talk server and a chat server.