In a telecommunications network, a method of controlling a media gateway to handle a multiplex session, using a media gateway controller, the media gateway and the media gateway controller communicating using a standardised interface protocol, e.g. H.248, providing for a set of command messages each of which contains: a context field for identifying a context of the media gateway; a termination field for identifying one or more terminations of the media gateway; at least one descriptor for defining properties of the context; and optionally a package identity and at least one associated property. The method comprises, at the media gateway controller, generating a command message said structure and containing a multiplex descriptor including a package identifier, the package identifier identifying a package implemented at the media gateway for handling a multiplex session, sending the generated command message from the media gateway controller to the media gateway, and, at the media gateway, establishing the context identified in the message according to the specified package. Preferably, the multiplex descriptor includes at least one property which is one of a circuit assignment map and a contiguous or non-contiguous service type.