[H]

Hint 1 of 6:

Have you seen anything else that might require batteries?

Hint 2 of 6:

How about the penlight from the software store?

Hint 3 of 6:

Disassemble that and you'll find the other two batteries.

Hint 4 of 6:

But unfortunately, the batteries aren't charged.  Have you ever wondered how
the batteries in the phone managed to stay fresh for all that time?

Hint 5 of 6:

That's because the base of the portable phone has a built-in charger.

Hint 6 of 6:

Put the batteries in the handset and then put the handset back.