A callout is when you take a delivery job and then come off it. If you don't want to contest it, tap I do not dispute on the card and it closes with the effect applied.
Nothing is created in the first 15 minutes. After that, the timing decides what happens. A callout more than 24 hours before the pickup closes without affecting your reliability rating. Inside 24 hours it affects the rating, and it affects it more the closer to the pickup time the job was.
If you do want to contest it, the callout arrives as a card in your Inbox, with a chat under it. Open the card and write what happened. The image button next to the message box adds a photo that backs it up. A callout can end up reduced or unchanged, and one with no reply closes unchanged.