For android:
Please find your call history in Contacts - Call history.
If you miss this option, please upgrade imo then retry.
For iOS:
You may check your call history in
1) imo - Contacts - Call history
or 2) your iPhone's call history
Tips:
You may press on the call record for over 1s -Delete. Please notice that the deleted call record can not be recovered.
Click on the "..." button at the top corner to delete selected records or delete all records.