Signal – Private Messenger received an update yesterday that enabled video calling for the first time. This version also brings CallKit support to the platform, which may be concerning to some privacy-conscious users.
According to the Open Whisper Systems blog post, privacy-conscious users you can opt-out of the CallKit features.
moxie0, Writing on Open Whisper Systems Blog
Enabling the new calling features on iOS will also allow you to take advantage of CallKit in iOS 10. This allows incoming Signal calls to be answered with one touch, directly from the lock screen. Keep in mind, this only takes effect when both parties are using the new calling system.
CallKit offers a native calling experience to VoIP apps like Signal. As well as being able to answer calls directly from your lock screen, you’ll also see Signal calls in the system’s “Recent Calls” list. This is because iOS treats CallKit calls like any other call, however that also means some information will be synced to iCloud if enabled. This information includes who you called and how long you talked.
If you decide that’s not for you, you can opt-out of the CallKit features at any time in Settings > Advanced > Use CallKit, while continuing to use the rest of the new calling system.