It appears that the phone call related APIs that Microsoft removed from Windows 10 in the Fall Creators Update have been added back again in Windows 10 Redstone 5 Build 17650, which rolled out to the 'Skip Ahead Insiders' channel. This could be another indication of an approaching Surface Phone release date.
https://www.notebookcheck.net/Microsoft-adds-mobile-APIs-back-into-Windows-10-possibly-for-Surface-Phone.300682.0.html