You can purchase by pressing *675# menu, see the screenshot below as your guide,
Step 1: Press *675# and select option 3 where it says "Happy hour, Voice & SMS."
Step 2: Select option 1 for Happy Hour voice bundle priced at K2.40. Then, select option one to confirm the purchase of this bundle,
Step 3: After you successfully purchase this bundle, you will receive the following notifications,