well like i said it was an old phone i had the issue with so its not unique to the n900 (although it was probably still a nokia), and i wouldn't hold out too much hope for a bugfix for this one in a million bug on a largely unsupported device, but it is easily documented and reproducable so if you wish to try feel free.

have you also tried putting one or two random numbers at the end of each contact number? cant say for sure about the phone system where you are, but any extra numbers after a working number are usually ignored by the telephone system, but possibly not by the phonebook.