You need to create a formula text field on the Task object and use the below formula.
Step 1. Navigate to Setup
Step 2. Click on Object Manager and then Activity
Step 3. Click on Fields & Relationship and New to create the field
Step 4. Select Formula and click on the Next button
Step 5. Type any label in the Field Label field. The Field Name will automatically be filled in
Step 6. Select Text as Formula Return Type and click on the Next button
Step 7. Use the formula below and click on the Check Syntax button available at the bottom left of the editor to validate the entered formula
IF
(ISBLANK( CallDurationinSeconds),NULL,
LPAD(TEXT( FLOOR(CallDurationinSeconds /60 / 60)),2,"0") + ":" +
LPAD(TEXT(FLOOR( (CallDurationinSeconds)/60) - FLOOR(CallDurationinSeconds /60 / 60)*60),2,"0") + ":" +
LPAD(TEXT( FLOOR(MOD((CallDurationinSeconds)/60,1)*60)),2,"0")
)
Step 8. In the Blank Field Handling section, select Treat blank fields as blanks and click on the Next button
Step 9. Select the appropriate Profiles for the Field Level Security and click on Next button
Step 10. Select the appropriate Page Layouts and click on the Save button
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