The phone number is missing in the Google Places API because Google Maps doesn’t have a phone number for that particular place. If you request the details for a place with a phone number on Google Maps, it will be returned.
Take a closer look at ChIJ8UadyjeuEmsRDt5QbiDg720. It’s an office building called “Workplace 6” — notice that the details say it is of type premise. If you visit the Google Maps URL in the url field (https://maps.google.com/?q=Workplace+6&ftid=0x6b12ae37ca9d46f1:0x6defe0206e50de0e) you’ll notice there’s no phone number listed there either.
The other Place ID, ChIJN1t_tDeuEmsRUsoyG83frY4, is for one of the businesses located inside that building (Google). If you compare the address_components or formatted_address you’ll see they don’t have quite the same address (Google’s address is floor 5 of that building).
In general, any field of a place might be absent. This happens if Google Maps doesn’t know it, or if it simply doesn’t apply (e.g. you won’t find opening_hours on France). As the documentation for Place Details Results say:
Each result may contain the following fields:
[Emphasis mine.]