The problem annoying many iPhone users specially locked one, so bellow solutions for this problem, follow them and you will get the name displaying back again.
The solution that worked for me and for many other with similar problem is very simple, just by switching the language:
- go to settings>general>international.
- change language to any other language for example british english.
- Then switch back back to your original language.
- Go to setting-General-International->Region format.
- And select your country.
- Then hit OK and restart your iPhone.
If that solution doesn't work with you, you'll have to add +country prefix to your contacts number, for example add +1 for contacts located on the united states.
If for any reason the two above does not work, a solution for Verizon users is by contacting over-the-air programming number.
- Just dial *228, then press 1 for "Program or activate your phone".
- Wait for the call to disconnect, and you'll receive setting update configuration.
- Close recently used applications by double taping home button and long press running application.
- Wait four minutes or more.
- Now restart your iPhone and see if it's fixed.
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