Android use the default "Calling app" when the user wants to make a call. MS Teams is one such "Calling app": the first time you install any other than the default shipped with your phone, you (the user) are presented a choice to choose which one you want to use. After that, Android remembers your choice.

This means in this case, MS Teams was configured as the default "Calling app" and the issue could have been prevented at 2 level:

- at the Android level, if the user dials an emergency number, don't use the default "Calling app" and use a special "safe" calling app to ensure the call succeed even if a user-installed app is misbehaving.

- at the MS Teams level, allow emergency calls to succeed even if the user isn't logged in (or any other reason that could prevent an emergency call to be made, really).

As for WhatsApp, at least on my device, it's not a "Calling app", and as such cannot override the default calling app. I don't have Signal installed to check.

How exactly do you check your "calling apps"? I'm on Android 12 and have a list of "Phone apps". However, the only other app listed there (besides "Phone") is a VOIP app I specifically installed to make alternative phone calls with. Signal, WhatsApp, Telegram and not listed, even though I've used all of them to call other users on the respective apps.

First: I used "Calling apps" because that's how it's displayed on my Android 8 phone. The actual naming isn't consistent across Android versions, so yours is probably named "Phone apps". That's located in the settings, and again the exact way to access it varies according Android versions, manufacturer and whatnot (which is an endless source of pain to guide end-users by the way).

To appear there, apps have to declare they are phone apps and handle the proper calls (an "Intent" in Android jargon) when the system receive a request to make a phone call. WhatsApp, Signal and Telegram do not do that: you are only able to initiate a call when already inside the app.

I don't know about Android 12, but on Android 11 it's in Settings > Apps an notifications > Defaults Apps. This is where you choose which app you want as default for phone, browser, messages, etc

> at the Android level, if the user dials an emergency number, don't use the default "Calling app" and use a special "safe" calling app to ensure the call succeed even if a user-installed app is misbehaving.

That doesn't make sense, the user has to pick the app to dial with before they have an interface to enter the number.

Assuming that your hypothesis is correct: No. Signal doesn't have that permission and I think whatsapp doesn't either, and neither signal nor whatsapp can be signed out in the first place.

I wouldn't go so far as calling my post a hypothesis, it was like a guess. Another guess, wouldn't the fact that neither WhatsApp nor Signal are causing the same issues hint at MS Teams being the culprint?

Teams does the wrong thing, there's no question about that. But it's not clear to me that the Android core is unable to check that the phone app does the job, and has to trust it blindly.