Usually, ShippingAddress.phone is returned as "Invalid Request" if the user making API call is not the seller of the order or if the transaction is very old.
However for Germany site, by default, ShippingAddress.phone is returned as "Invalid Request" if the buyer is registered in Germany site.
To override this behavior, the seller needs to turn on the “Require Phone Number for shipping” option in MyeBay (My eBay > My Account > Site Preferences >Shipping Preferences > Require phone number for shipping).
In all other sites, this option is used to force the buyer to enter a phone number at the time of checkout. But for DE site, this option is also used to decide if ShippingAddress.phone has to be returned in the API response.