Both lists are in SharePoint, the only difference between the two of them is the amount of records. There are about 10 records in 'ADContacts' and 1900 in 'Phones 2'. I've also changed the delegation value to 2000 in case that might have been a factor.
This shouldn't be an issue with delegation or anything, but may be a disambiguation issue or something with how that column is cached.
Can you add a new empty Gallery, use 'Phones 2' as the Items property, and the confirm that ThisItem.Extension is populating in a label (or any applicable control)? It may help to ensure the column is actually coming to PowerApps with that friendly name and can be referenced explicitly.