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 / notification options ( sound
/ pop up / banner / lock screen, etc.) in phone
setting.
4. Ensure imo tone is active.
You may modify it in [Me] page - Settings
- Notifications.
5. Keep phone on sound mode. Turn up the
volume.
6. Switch network.
7. Reinstall the latest version.
Note:
For Transsion ITEL devices, please turn
off Do Not Disturb mode then try again.