A method for preventing illegitimate change of an identification number of a mobile terminal, the method comprising: if change of an identification number is requested, determining whether authentication information is stored; if the authentication information is stored, encoding (S212, S312) the stored authentication information; determining whether the previously stored first authentication information exists; if the previously stored first authentication information does not exist, storing the identification number and storing the encoded stored authentication information as a first authentication information; if the previously stored first authentication information exist, determining (S216, S313) whether the encoded stored authentication information corresponds to a previously stored first authentication information; if the encoded stored authentication information corresponds to the previously stored first authentication information, storing (S218, S315) an identification number; and if the encoded stored authentication information does not correspond to the previously stored first authentication information, performing (S219, S319) error handling, wherein the first authentication information is stored in an One Time Programmable, OTP, area in which information stored once cannot be permanently changed and erased, and wherein the authentication information is stored in a flash area in that data stored is retained in case of power off, said method further comprising receiving and storing a code from the authentication data server when the new authentication information is received and stored, wherein the step of encoding (S212, S312) the stored authentication information further comprises the steps of: transmitting the stored authentication information to an authentication data server and receiving a code corresponding to the transmitted authentication information; determining whether the received code corresponds to the stored code; if the received code corresponds to the stored code, encoding (S212, S312) the stored authentication information; and if the received code does not correspond to the stored code, performing (S219, S319) error handling. if change of an identification number is requested, determining whether authentication information is stored; if the authentication information is stored, encoding (S212, S312) the stored authentication information; determining whether the previously stored first authentication information exists; if the previously stored first authentication information does not exist, storing the identification number and storing the encoded stored authentication information as a first authentication information; if the previously stored first authentication information exist, determining (S216, S313) whether the encoded stored authentication information corresponds to a previously stored first authentication information; if the encoded stored authentication information corresponds to the previously stored first authentication information, storing (S218, S315) an identification number; and if the encoded stored authentication information does not correspond to the previously stored first authentication information, performing (S219, S319) error handling, wherein the first authentication information is stored in an One Time Programmable, OTP, area in which information stored once cannot be permanently changed and erased, and wherein the authentication information is stored in a flash area in that data stored is retained in case of power off, said method further comprising receiving and storing a code from the authentication data server when the new authentication information is received and stored, transmitting the stored authentication information to an authentication data server and receiving a code corresponding to the transmitted authentication information; determining whether the received code corresponds to the stored code; if the received code corresponds to the stored code, encoding (S212, S312) the stored authentication information; and if the received code does not correspond to the stored code, performing (S219, S319) error handling.