Methods of identifying a user plane identifier of a user device by a monitoring probe in communication with a network device, the monitoring probe, and the system using the same are disclosed. In an embodiment, the monitoring probe includes a monitor, a comparator and a user plane identifier output. The monitor monitors a network device for receipt of a first control plane message comprising a first control plane identifier and a user device identifier, and monitors the network device for receipt of a second control plane message comprising a second control plane identifier and a user plane identifier. The comparator compares the first with the second control plane identifier to determine whether the control plane identifiers correspond. The user plane identifier output outputs the user plane identifier being identified as that of the user device, when the control plane identifiers correspond.