Issues relating to your apps not working and phone cannot make calls: AIRPLANE MODE?
From your home screen swipe down from the top to open a quick menu screen check and verify that AIRPLANE mode is off, Data should be on, Sync should be on, WiFi on. Verify WiFi is connected, if there is an exclamation point ! following the WiFi connection you may have to restart your modem.
STORAGE & MEMORY?
If you have MPEG4 Video, MPEG4 Music, Adobe PDF Files saved to the phone's memory check the STORAGE under SETTINGS to verify that your not exceeding the phone's storage capacity. This will affect your apps and the phone's overall performance. Recommend moving stored files to the external SD card.
INSTALLED APPS NOT OPENING?
Sometimes errors occur when downloading the app, Uninstall then reinstall the apps once more. If still not opening in rare occasions the App is not compatible with the ZTE phone. When you visit the play store confirm that the app has been authorised "PLAY PROTECT" By Google and never download UNAUTHORIZED APPS it may contain SPYWARE and Malicious intentions.
The information listed above is from my personal experiences using android driven devices.
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=== Update (12/21/2019) ===
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Suggest you do a BACKUP & RESTORE. Just in case some data is lost you should first screenshot or write down all of your contacts/phone numbers and the name of your apps previously installed. From your home screen swipe down then click on the cog gear looking icon on the upper right of the screen. This will open the settings menu scroll up and click on BACKUP & RESTORE. Backup data on, Backup account enter the email account associated with your phone, automatic restore on, click on FACTORY DATA RESET. Before confirming make sure that you know the email account and password that is associated with your phone. This information will be required when the phone RESETS & RESTORES.
ZTE USER
Issues relating to your apps not working and phone cannot make calls: AIRPLANE MODE?
From your home screen swipe down from the top to open a quick menu screen check and verify that AIRPLANE mode is off, Data should be on, Sync should be on, WiFi on. Verify WiFi is connected, if there is an exclamation point ! following the WiFi connection you may have to restart your modem.
STORAGE & MEMORY?
If you have MPEG4 Video, MPEG4 Music, Adobe PDF Files saved to the phone's memory check the STORAGE under SETTINGS to verify that your not exceeding the phone's storage capacity. This will affect your apps and the phone's overall performance. Recommend moving stored files to the external SD card.
INSTALLED APPS NOT OPENING?
Sometimes errors occur when downloading the app, Uninstall then reinstall the apps once more. If still not opening in rare occasions the App is not compatible with the ZTE phone. When you visit the play store confirm that the app has been authorised "PLAY PROTECT" By Google and never download UNAUTHORIZED APPS it may contain SPYWARE and Malicious intentions.
The information listed above is from my personal experiences using android driven devices.