First, please try to clear the Teams desktop client and mobile client cache.
Clear desktop client cache:
Step 1. If Teams is still running, right-click the Teams icon in the taskbar, and then select Quit.
Step 2. Press Win + R to open the Run box. Then type %AppData%\Microsoft\teams\ in the box and press Enter to run it.
Step 3. In the pop-up File Explorer window, double-click the Cache folder and then delete all files in the folder.
Step 4. Restart Teams.
If after clearing the cache, the contact still does not show the name, please add the country code to the contact's phone number.
If the country code doesn't exist, Teams ignores the contact and displays the caller's public caller ID list.
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