Please follow steps as below:
1. Keep imo stay on your phone background.
2. Remove imo from the lock screen cleanup list
of your phone and security apps.
3. Turn on imo call permissions ( sound / pop up
/ banner / lock screen, etc.) in phone permission
and battery setting, enable imo lay over other
apps.
Please tap the link below and nevigate to your
phone setting: [ Detailed Settings Here ]
4. Ensure imo tone is active.
You may modify it in profile page - settings
- notifications.
5. Keep phone on Sound mode.
Turn up the volume.
6. Switch your network in case of network issue.
7. VPN may help if it frequently happens in your
area.
8. Upgrade imo then retry.
You may tap the update button on Google
Play Store or App Store.