A system for activating SIMs, wherein the system comprises more SIMs than will be authorised for use on a mobile network, the system comprising: a plurality of SIMs, each of said SIMs being configured to provide an authentication identifier such that the plurality of SIMs provide a plurality of authentication identifiers having values within a defined set of values, wherein one or more of said plurality of authentication identifiers is reused amongst said plurality of SIMs such that one of said plurality of SIMs may provide the same said authentication identifier as another of said plurality of SIMs; and a SIM authorisation server, configured to receive a said authentication identifier from a SIM requesting activation, determine whether said authentication identifier has been used to activate a SIM and, in response to a successful activation, to provide a traffic identifier to said SIM requesting activation and wherein said traffic identifier is not within said defined set of values, wherein said traffic identifier enables said SIM requesting activation to communicate traffic over said mobile network; and wherein, in response to failure of said activation, said SIM requesting activation is configured to: start an algorithm that is configured, in response to determining that the authentication identifier is within the defined set of values, to create a different authentication identifier from at least the said authentication identifier previously transmitted to the SIM authorisation server, wherein the said authentication identifier is assigned to a pool of authentication identifiers, the said authentication identifier comprising a pool identifier digit and a plurality of lower-order digits randomly assigned to the SIM requesting activation at manufacture, said pool being one of a plurality of pools of authentication identifiers and each pool comprising a set of unique authentication identifiers; and further wherein said different authentication identifier is assigned to a further pool of the plurality of pools, and said different authentication identifier is generated by incrementing or decrementing said pool identifier digit; and provide said different authentication identifier to said SIM authorisation server, the different authentication identifier having a value within the defined set of values a plurality of SIMs, each of said SIMs being configured to provide an authentication identifier such that the plurality of SIMs provide a plurality of authentication identifiers having values within a defined set of values, wherein one or more of said plurality of authentication identifiers is reused amongst said plurality of SIMs such that one of said plurality of SIMs may provide the same said authentication identifier as another of said plurality of SIMs; and a SIM authorisation server, configured to receive a said authentication identifier from a SIM requesting activation, determine whether said authentication identifier has been used to activate a SIM and, in response to a successful activation, to provide a traffic identifier to said SIM requesting activation and wherein said traffic identifier is not within said defined set of values, wherein said traffic identifier enables said SIM requesting activation to communicate traffic over said mobile network; and wherein, in response to failure of said activation, said SIM requesting activation is configured to: start an algorithm that is configured, in response to determining that the authentication identifier is within the defined set of values, to create a different authentication identifier from at least the said authentication identifier previously transmitted to the SIM authorisation server, wherein the said authentication identifier is assigned to a pool of authentication identifiers, the said authentication identifier comprising a pool identifier digit and a plurality of lower-order digits randomly assigned to the SIM requesting activation at manufacture, said pool being one of a plurality of pools of authentication identifiers and each pool comprising a set of unique authentication identifiers; and further wherein said different authentication identifier is assigned to a further pool of the plurality of pools, and said different authentication identifier is generated by incrementing or decrementing said pool identifier digit; and provide said different authentication identifier to said SIM authorisation server, the different authentication identifier having a value within the defined set of values start an algorithm that is configured, in response to determining that the authentication identifier is within the defined set of values, to create a different authentication identifier from at least the said authentication identifier previously transmitted to the SIM authorisation server, wherein the said authentication identifier is assigned to a pool of authentication identifiers, the said authentication identifier comprising a pool identifier digit and a plurality of lower-order digits randomly assigned to the SIM requesting activation at manufacture, said pool being one of a plurality of pools of authentication identifiers and each pool comprising a set of unique authentication identifiers; and further wherein said different authentication identifier is assigned to a further pool of the plurality of pools, and said different authentication identifier is generated by incrementing or decrementing said pool identifier digit; and provide said different authentication identifier to said SIM authorisation server, the different authentication identifier having a value within the defined set of values