How do I add a phone number to a buyer's label?

[unicornsalesxx]

I'm shipping international package and they asked me to put their phone number on the label. I don't see any option on ebay's label making page to add their number. Should I add it to the "comment" section for the item? Or write it down below their address after printing it? 

[disneyshopper]

I think either option would work.