The operator name is dependent of handset. The intresting fact is every phone manufactured will have the list of mobile operators available all over the world along with the MNC or network code. The procedure to find in Sony ericsson phones is Menu->settings->connectivity->mobile networks->preferred network->new network.
Here u can see names of all operators in the world. whenever u enter a roaming area, the name will be displayed according to the MCC-MNC available there. Hope this clarifies.