On iPhone models without the Home button, follow the steps below:
Step 1: Press and hold the Volume Up button then release.
Step 2: Press and hold the Volume Down button then release.
Step 3: Press and hold the Power button until the Apple logo appears on the screen.
On iPhone
models with a Home button, follow the steps below: Press and hold the Power button + Home button at the same time until you see the Apple logo displayed on the screen.
On the iPhone, Apple adds a feature called Dial Assist (dial assist) that automatically determines the international or local prefix when dialing. Go to Settings => Phone and check if the Dial Assist option is activated.
If you haven't activated the option and check if the contacts on your iPhone have displayed the username or not.
Duplicate contacts can also cause iPhone not to display names in contacts. Using 3rd party apps to search for and delete duplicate contacts can also help fix errors.
Here is a list of the top best apps to delete duplicate contacts on iPhone that you can refer to.
A little note is not to forget to sync contacts on iCloud. On your iPhone, go to Settings => iCloud => contacts and activate the option.
Good luck.