The invention relates to the field of Computer telephony Integration and in particular to the migration of the interface standardised according to ECMA-29 to the private branch exchange (PBX) via a CSTA gateway. The clients (C1-C4) in the data network subscribe to CTI Services and in this regard must be reachable by the CSTA gateway (CCGW). If this is not the case, because for example an error occurs in the data network, then the transmission of unnecessary messages should be prevented or stopped. The problem is solved in that the clients are connected via a server (XS) to the CCGW, wherein the server monitors the Status of the clients and acknowledges the messages addressed to an unreachable client with error messages. The server (XS) is preferably an XMPP server and the clients (C1-C4) are preferably XMPP clients. In order to permit transport of the CSTA messages via XMPP, info/query stanzas according to RFC3920 are used, since they have the advantage over message stanzas that they are acknowledged by the remote Station.