Generosity
Cover their call
Offer to pay for both sides so the person you're calling doesn't spend any credits.
Standard calls are split, each person spends 6 credits per 5-minute block. When you tap Cover their call next to the Call button, you're telling Mingle you'd like to pick up the whole tab: 12 credits per 5-minute block from your wallet, nothing from theirs.
The receiver sees a banner the moment the call connects and can either accept or choose "I'll pay my way". If the call gets extended, your offer keeps applying, you don't have to re-offer every 5 minutes.
You pay for both sides
When you choose to cover their call, you're charged for both sides (12 credits per 5-minute block) and they spend nothing, including any extensions. They can still choose to pay their own way.