The second option is to add a custom field to the invoice. However, you'll need to manually enter their phone numbers every time. Consider checking out this article for the detailed steps on how: How to add custom fields to invoices.
I'll pass along your suggestion to our Product Developers. They're always looking for ideas to consider on how to improve QBO. In the meantime, I recommend visiting our Blog site. This is where we share recent happenings and future developments, such as updates to newly added features.
Reach out to me in the comment section below if you have any other concerns. I'm always here to answer them. Have a great day!