Methods and systems for sharing data usage by a plurality of SIM cards at a plurality of network devices are disclosed. Each of the plurality of network devices is capable of housing at least one SIM card from the plurality of SIM cards. The method comprises the steps of determining the total data usage allowance. A remaining total data usage allowance is determined by either retrieving it from a carrier server or by subtracting aggregated data usage information from the total data usage allowance. The method further comprises restricting use of one or more of the plurality of SIM cards when the remaining total data usage allowance is below a first threshold until the remaining total data usage allowance is above the first threshold. The total data usage allowance is changed to a first value periodically or according to an administrator's instruction.

An operation method of a session management function (SMF) is provided. The method includes the steps of transmitting a request message to a user plane function (UPF) on the basis of the UPF and a session configuration, receiving a response message from the UPF, determining whether there is a lost usage amount on the basis of the response message, and, when there is a lost usage amount, transmitting recovered usage amount information to an online charging system (OCS), wherein the response message may include current usage amount information and cumulative usage amount information of the UPF.