This is listed as a known issue in the 10.3 release notes.
https://developer.apple.com/library/content/releasenotes/General/RN-iOSSDK-10.3/
openURL
When a third party application invokes openURL: on a tel://,
facetime://, or facetime-audio:// URL, iOS displays a prompt and
requires user confirmation before dialing.
It is also listed in the Security content of the 10.3 update, so I’m assuming this a new feature that is here to stay.
An issue existed in iOS allowing for calls without prompting. This
issue was addressed by prompting a user to confirm call initiation.