A method for verification of a mobile telephone number in order to determine whether a positioning service forming part of a third party service in a mobile radio communications system should be performed in relation to the mobile telephone number, the method comprising: storing a mobile telephone number and a corresponding mobile subscriber identity number in a database of a verification apparatus; and being characterized by the steps of: receiving, in the verification apparatus, a request for verification of the mobile telephone number; sending, from the verification apparatus in response to the verification request, an IMSI query comprising information on the mobile telephone number and requesting a mobile subscriber identity number registered in association with the mobile telephone number in a home location register; receiving, in the verification apparatus, the mobile subscriber identity number associated with the mobile telephone number in the home location register; comparing, in the verification apparatus, the stored mobile subscriber identity number and the received mobile subscriber identity number; and, if the comparison shows that the received mobile subscriber identity number corresponds to the stored mobile subscriber identity number, initiating the performance of the positioning service in relation to the mobile telephone number, and if the comparison shows that the received mobile subscriber identity number does not correspond to the stored mobile subscriber identity number, sending a message to the mobile telephone number via the mobile radio communications system, the message indicating that delivery of the third party service has been terminated. storing a mobile telephone number and a corresponding mobile subscriber identity number in a database of a verification apparatus; and being characterized by the steps of: receiving, in the verification apparatus, a request for verification of the mobile telephone number; sending, from the verification apparatus in response to the verification request, an IMSI query comprising information on the mobile telephone number and requesting a mobile subscriber identity number registered in association with the mobile telephone number in a home location register; receiving, in the verification apparatus, the mobile subscriber identity number associated with the mobile telephone number in the home location register; comparing, in the verification apparatus, the stored mobile subscriber identity number and the received mobile subscriber identity number; and, if the comparison shows that the received mobile subscriber identity number corresponds to the stored mobile subscriber identity number, initiating the performance of the positioning service in relation to the mobile telephone number, and receiving, in the verification apparatus, a request for verification of the mobile telephone number; sending, from the verification apparatus in response to the verification request, an IMSI query comprising information on the mobile telephone number and requesting a mobile subscriber identity number registered in association with the mobile telephone number in a home location register; receiving, in the verification apparatus, the mobile subscriber identity number associated with the mobile telephone number in the home location register; comparing, in the verification apparatus, the stored mobile subscriber identity number and the received mobile subscriber identity number; and, if the comparison shows that the received mobile subscriber identity number corresponds to the stored mobile subscriber identity number, initiating the performance of the positioning service in relation to the mobile telephone number, and if the comparison shows that the received mobile subscriber identity number does not correspond to the stored mobile subscriber identity number, sending a message to the mobile telephone number via the mobile radio communications system, the message indicating that delivery of the third party service has been terminated.