That's why

Context: The salesman, who missed the prospective buyer's call, is calling him back and asking about the purpose of the call.

The salesman: I just missed your call. By the way, may I ask what the purpose of your call was?

Prospective buyer: I want to enquire about the product. that's why I called you.

Please check them.

I am wondering if "May I ask what the purpose of your call was?" and "that's why" are correctly used in the context.