A method of using accounting certificates to allow a subscriber of a home network to purchase services or products via a mobile communications network, characterised in that the method comprises: sending a request for an accounting certificate from a subscriber's mobile terminal to a Network Application Function node in a visited network, the Network Application Function node comprising a Public Key Infrastructure portal which uses shared secret keys to communicate with said subscriber's mobile terminal: upon receipt of the request at said Network Application Function node, (communicating between the Public Key Infrastructure portal and a Bootstrapping Server Function of the subscriber's home network to obtain at the Public Key Infrastructure portal a shared secret previously agreed between the Bootstrapping Server Function and the subscriber's mobile terminal and an identity of the responsible online charging system within the home network, and using said identity to send an authorisation request from said Network Application Function node to said online charging system; at the online charging system, making a decision on said request based upon the subscriber's account data, and returning either an accept or deny service request message to said Network Application Function node; in the event that an accept service request message is received by said Network Application Function, sending the requested accounting certificate to the subscriber terminal; and sending the accounting certificate from the subscriber terminal to the provider of a product or service to be paid for. sending a request for an accounting certificate from a subscriber's mobile terminal to a Network Application Function node in a visited network, the Network Application Function node comprising a Public Key Infrastructure portal which uses shared secret keys to communicate with said subscriber's mobile terminal: upon receipt of the request at said Network Application Function node, (communicating between the Public Key Infrastructure portal and a Bootstrapping Server Function of the subscriber's home network to obtain at the Public Key Infrastructure portal a shared secret previously agreed between the Bootstrapping Server Function and the subscriber's mobile terminal and an identity of the responsible online charging system within the home network, and using said identity to send an authorisation request from said Network Application Function node to said online charging system; at the online charging system, making a decision on said request based upon the subscriber's account data, and returning either an accept or deny service request message to said Network Application Function node; in the event that an accept service request message is received by said Network Application Function, sending the requested accounting certificate to the subscriber terminal; and sending the accounting certificate from the subscriber terminal to the provider of a product or service to be paid for. upon receipt of the request at said Network Application Function node, (communicating between the Public Key Infrastructure portal and a Bootstrapping Server Function of the subscriber's home network to obtain at the Public Key Infrastructure portal a shared secret previously agreed between the Bootstrapping Server Function and the subscriber's mobile terminal and an identity of the responsible online charging system within the home network, and using said identity to send an authorisation request from said Network Application Function node to said online charging system; at the online charging system, making a decision on said request based upon the subscriber's account data, and returning either an accept or deny service request message to said Network Application Function node; in the event that an accept service request message is received by said Network Application Function, sending the requested accounting certificate to the subscriber terminal; and sending the accounting certificate from the subscriber terminal to the provider of a product or service to be paid for.