1. A method of invoking a feature in a switching system having an internal numbering plan in a telecommunications network having a network numbering plan, comprising the steps of: receiving at the switching system a first symbol sequence that is included in the internal numbering plan; in response to receiving the first symbol sequence, interpreting the received first symbol sequence at the switching system to be a feature access code of a feature of the switching system; receiving at the switching system a second call-control symbol sequence, different from the first symbol sequence, that is included in the network numbering plan and that is assigned, within the network numbering plan, to an extension of the switching system; in response to receiving the second symbol sequence, interpreting the received second symbol sequence at the switching system to be an equivalent of said feature access code of said feature of the switching system; and in response to the interpreting of either one of the received first symbol sequence and the received second symbol sequence, invoking said feature in the switching system.