I found on Symbian page:

Can I access the IMEI number of a phone from code on Symbian OS?

The IMEI number is accessible through a native ETel API. Unfortunately, for C++ developers this is not a publicly released API but is part of the advanced GSM API to which only licensees have access. However the API is accessible from Java code using the methods of JTAPI mobile, as follows:

JtapiPeer peer = JtapiPeerFactory.getJtapiPeer("com.symbian.javax.telephony.EpocJtapiPeer"
provider = peer.getProvider("EpocGsmMobileProvider"
Address[] addresses = provider.getAddresses();
origAddress = addresses[0];
println("Host number is " + origAddress.getName());
Terminal[] terminals = provider.getTerminals();
origTerminal = (MobileTerminal) terminals[0];
println("IMEI number is " + origTerminal.getTerminalId());