A network entity comprises an interworking function. The network entity is between an endpoint and a registrar in a telecommunications network. A message is received for the registrar from the endpoint, the message comprising data identifying an endpoint characteristic of the endpoint. A message is received for the endpoint. A processing action to be used to process the received message for the endpoint is determined based on the data identifying the endpoint characteristic of the endpoint comprised in the received message for the registrar from the endpoint. The received message for the endpoint is processed in accordance with the determined processing action.