Arranging internal data connections of office system

A method of establishing a data connection in a telecommunications system which comprises an office network and an operator network and a local area network between them. Data transmission between the office network and a public land mobile network is adapted together in the operator network. The office network comprises an interworking function to which a signalling connection from a call control entity is arranged, and the call control entity detects the establishment of an internal data connection in the office network, the data connection using a data protocol according to a mobile system. Data connections according to the data protocol and coming from a radio access gateway are adapted to the data protocol according to the office network in response to the call control entity detecting an internal data connection in the office network, the second party of the data connection being a mobile system terminal.