Authorizing access between communication networks

A computing device comprising:

• one or more hardware processors;
• one or more computer-readable storage media storing a program which, when executed under the influence of the one or more processors, implements a method comprising:
• receiving a call setup request from a user terminal of the user at a first network element of the first packet-based communication network, the authorisation request comprising a first user identity;
• responsive to the call setup request, transmitting a request to create a second user identity from the first network element to a second network element of the second packet-based communication network effective to cause:
• the second network element to create the second user identity for use in the second packet-based communication network, the second user identity being derivable from the first user identity according to a predetermined rule;
• and
• store the second user identity 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.