The CNetworkCellMonitor class shown in this code snippet monitors the network and cell a mobile device is registered on. It will report the Mobile Country Code, the Mobile Network Code and the CellId to an observer class.
Following is my contribution on how to retrieve IMEI and IMSI on S60 3rd edition. Along with that you can also use this code to retrieve network info (Cell Id, Location code). It can further be extended to retrieve other network information.