Arrangement for obtaining authentication key parameters in a cellular mobile telecommunications switching network

A method of obtaining authentication parameters for a mobile station on a mobile telecommunications call served by a first mobile switching center (MSC) comprising the steps of:

• determining that Visitor Location Register (VLR) data stored in said first MSC for said mobile station has fewer than a predetermined parameter, n, of authentication parameter sets available for said mobile station, n having a value of at least 2;
• responsive to said determining, requesting a second MSC which stores HLR data of said mobile station to provide a plurality of sets of authentication parameters for said mobile station;
• responsive to receiving the request for authentication parameter sets, accessing a first processor means of said second MSC for storing Home Location Register (HLR) data of said mobile station, said HLR data comprising an authentication key of said mobile station;
• from said first processor means, using said authentication key, accessing a second processor means of said second MSC for obtaining authentication parameters;
• obtaining a plurality of sets of authentication parameters from said second processor means;
• and
• transmitting said plurality of sets of authentication parameters to said first MSC for storing said sets in said VLR data for said mobile station.