As a Stakeholder, how am I notified when someone updates an incident that I'm subscribed to?

PagerDuty will contact you by the contact methods that you have in your notification rules. It will trigger notifications for an email, SMS, push for iOS and Android. However, you will not receive a phone call notification for an updated incident that you are subscribed to.

For more information on how to add and notify subscribers, see
Add and Notify Subscribers