A test system for testing a pair of telecommunications access networks, e.g. as part of transfer engineering. The system comprises a test head for connection, via an access arrangement, to first and second access networks. In a first phase, the access arrangement is arranged to connect the test head to the first access network for sending a first request from the tester to, for example, a CLI server via the first access network. The first request comprises a parameter associated with a user line. The tester is arranged to receive in response to the first request a first identifier from the server. In a second phase, the access arrangement is arranged to connect the test head to the second access network for sending a second request from the test head through the second access network to, for example, a CLI server. The second request also comprises the parameter. The tester is arranged to receive, in response to the second request, a second identifier from the target server and is arranged to carry out a comparison operation involving the first and second identifiers received in response to the two requests. A mismatch can indicate a fault or misconnection in either access network or a problem with network configuration data.