Method for using both a primary telephone number and a secondary telephone number in relation to a subscriber to a telecommunications network, the telecommunications network being a mobile communications network, and the primary telephone number and the secondary telephone number being MSISDN telephone numbers, wherein, by means of the telecommunications network, a communication connection can be established between the subscriber and a further subscriber, the subscriber and the further subscriber being connected to a core network of the telecommunications network via an access network of the telecommunications network, wherein the telecommunications network has or is connected to or has access to a caller identity unit or caller identity functionality, wherein the method comprises the following steps for establishing a communication connection between the subscriber and the further subscriber: - in a first step, the primary telephone number is used between the subscriber and the caller identity unit or caller identity functionality, - in a second step, the secondary telephone number is used between the further subscriber and the caller identity unit or caller identity functionality, wherein the communication connection is established between the subscriber and the further subscriber in that a call setup is performed at the subscriber or initiated at the subscriber, the first telephone number being used initially - during the first step - and the secondary telephone number being used later in time - during the second step - as a caller ID of the subscriber used towards the further subscriber, the secondary telephone number being recorded in a home location register, HLR, of an operator of the telecommunications network, a further communication connection being established between the subscriber and the further subscriber on the basis of the secondary telephone number recorded in the home location register in the event of a return call from the further subscriber. wherein, by means of the telecommunications network, a communication connection can be established between the subscriber and a further subscriber, the subscriber and the further subscriber being connected to a core network of the telecommunications network via an access network of the telecommunications network, wherein the telecommunications network has or is connected to or has access to a caller identity unit or caller identity functionality, wherein the method comprises the following steps for establishing a communication connection between the subscriber and the further subscriber: - in a first step, the primary telephone number is used between the subscriber and the caller identity unit or caller identity functionality, - in a second step, the secondary telephone number is used between the further subscriber and the caller identity unit or caller identity functionality, - in a first step, the primary telephone number is used between the subscriber and the caller identity unit or caller identity functionality, - in a second step, the secondary telephone number is used between the further subscriber and the caller identity unit or caller identity functionality, wherein the communication connection is established between the subscriber and the further subscriber in that a call setup is performed at the subscriber or initiated at the subscriber, the first telephone number being used initially - during the first step - and the secondary telephone number being used later in time - during the second step - as a caller ID of the subscriber used towards the further subscriber, the secondary telephone number being recorded in a home location register, HLR, of an operator of the telecommunications network, a further communication connection being established between the subscriber and the further subscriber on the basis of the secondary telephone number recorded in the home location register in the event of a return call from the further subscriber.