Methods and systems of authorizing a user of a first packet-based communication network to access a second packet-based communication network are disclosed. A call setup request is received from a user terminal of the user at a first network element of the first packet-based communication network, the authorization request comprising a first user identity. Responsive to the call setup request, a request is transmitted to create a second user identity from the first network element to a second network element of the second packet-based communication network. The second network element creates the second user identity for use in the second packet-based communication network. The second user identity is derivable from the first user identity according to a predetermined rule. The second user identity is stored in the second packet-based communication network for use with establishing a call associated with the call setup request over the second packet-based communication network.