Unable to Log Into Teams on iPhone. Error: Sign-in Error

It looks like the issue was caused by Teams not applying an InTune policy. We're using InTune to manage Teams. For some reason, teams wasn't applying the organization's data management policy and was getting blocked. Teams wasn't showing the user that this app will be managed and needs to be restarted.

The resolution was to receive a message in Teams and not view it. Eventually, the user will receive an email stating that a teammate is trying to reach you in Microsoft Teams. Within that email, there is a link to 'Reply in Teams'. If the user opens that link on their mobile device, it correctly initiates data management policy and manages Teams. The user was able to successfully log into Teams.