If you want to use call stickers, you can follow these steps:
1. Click to open a friend’s chat box
2. Click on the call button in the upper right corner
3. Enter the call page
4. Slide the icons arranged below
5. Select “Stickers”
6. Click on the sticker you want to display during the call
Please note that the sticker icon with the crown logo is only for use by premium members.