Have you thoroughly checked your phone book for duplicate numbers? The Nokia is very unforgiving in this dept, instead of just going "I have this number for two entries, may as well just give you the name of the 1st entry", it does what your symptoms are, which is to show nothing.
But if you definitely don't have duplicates (hey, I have a good idea, erase all entries in your phonebook and just create a single entry), then it would be probably a problem with your service provider.
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