Please follow the steps below：
1. Make sure Airplane mode is turned off:
When Airplane mode is turned on, you can't make or receive calls or text messages with your Zenfone. Accessing data services such as email or internet will be unavailable as well.
Swipe down from the top of the screen to access Quick Settings
or Tap Settings > Connections > Uncheck to turn off Airplane mode.
2. Check your SIM card settings:
Tap Settings > Connections > SIM card > Check the SIM card you want to use.
3. Make sure your device has at least 2 bars of signal strength.
A weak signal may make it difficult to make or receive calls.
4. Restart your device.
Press and hold the Power key for 2-3 seconds > Tap Restart
5. Backup your data and reset your device.
Note: Once the reset is performed, all the data, apps, settings and personal information will be deleted after the device initialization.
If the above steps can't resolve your issue, please contact the ASUS Product Support for assistance.
This information may not apply to all products from the same category/series. Some screenshots and operations may be different depending on the software versions.
