Systems and methods are disclosed for validating an association of a telephone number with a cellular communication device (e.g., validating an assignment of an MSISDN to an IMSI of the cellular communication device). For example, the validating may be performed as part of a process of registering the cellular communication device with a registration server for participation in online communication sessions. In other examples, the validation may be performed for purposes other than registration for participation in online communication sessions. Various mechanisms may be available for performing the validation. The registration server may determine the available mechanisms, and may provide to the cellular communication device an ordered list of mechanisms according to a preferred order. The cellular communication device may improve efficiency of validation by attempting the mechanisms in the preferred order.