I may be wrong - your point of view may be the more prudent, but I think of it this way.
The ‘buyer’ you are trying to contact, is the person who bought from the dropshipper and is therefore not your buyer and often as not, not an Amazon customer.
Phoning this buyer re a Delivery is exactly why Amazon provide the ‘actual’ buyer’s phone number.
Hence, as long as you refrain from mentioning “Phone Call” in the Amazon Message log and speak with the end recipient only about delivery’ then you abide by Amazon Policy, even if they consider that person to be ‘their’ customer (even when they are not).
Clear as mud I think.