Although that is good common sense advice - this is Amazon and is the actual worst thing you could do. You WILL get banned forever if they find one comment relating to you phoning customers - especially when the customers aren’t yours. The customer gave their details to the dropshipper (if that is the correct therory- I believe so), so you have not been handed the persmission to use that data.
I know, I know I hate me for even saying that too but the OP is massively at even more risk of losing their account if Amazon get the slightest wind of a phone call being made. Again, even though that is perfectly reasonable outside of Amazon. There are several posts where the seller has stated such and this has prompted the ban - not the original issue.
I think the OP needs to kill off this line from the dropshipper POV (increase prices, send tracked, etc) or just lose the product for a while and re-list after the account is back in good standing and the frquency of orders are lower.
The reason there aren’t any phone numbers is that you’ve shipped the item - it removes the phone number instantly on dispatch.