The main reasons are:

1. Duplicate entries in contacts, the most common cause and unfortunately. The most awkward one to fix. If the phone finds the same number against more than one name, it doesn't know which to use so, perfectly sensibly, it defaults to showing the number. The only way to fix this is to remove duplicates from your contacts. Best way to do this would probably be PC Suite Contacts. When you search, use part of the number to avoid problems with formatting, searching for +60123456789 will not find +60 12 3456789, whereas searching for 789 would.

2. Using your international dial code instead of '+'. Not sure if this applies to all phones, but if you store your number using your county's international dial code (eg: 004412345678990 - WRONG) instead of the '+' sign (+4412345678990 - RIGHT), it can cause problems.

3. "(0)" area code prefixes. Only a problem if you synch with MS Outlook / PC Suite. When you specify a country in MS Outlook / PC Suite, it will insert "(0)" into your phone number after the country code +603123456789 becomes +60 (03) 23456789. You need to remove the (0).

4. 9 and 10 digit numbers. There has been one report of problems with 9 and 10 digit numbers where one size doesn't work with SMS and the other with calls. Not sure that anyone else has ever come across this.

5. Move all contacts from SIM Card to Phone Memory