> If you receive a phone call while locked presumably the phone can still access the address book to display the contact name and photo?
Just so you know, this is true on an iPhone, but NOT if the phone has NEVER been unlocked since reboot. If you get an SMS/call in this state, it will just show the number. It can't read the address book.
I think for iMessage, the actual messages are sent using APNS, so the message is in the push notification itself. Thus while you can see the message itself without unlocking, any older messages that are behind the Secure Enclave are inaccessible without keys.
This is correct. For example, when I connect my iPhone to my provided work wi-fi, and I get a Tinder notification. I can partially see the message on the lock screen (once Face ID authenticates), but as Tinder is blocked on the wi-fi if I want to read and respond in the app I have to pop to cellular.
If you receive a phone call while locked presumably the phone can still access the address book to display the contact name and photo?
And music playing apps can presumably access their database of music to play songs whilst the phone is locked?