It's clearly that your app does not have the Call Control permission. All you need to do is to add the missing permission and test it again.

@Phong Vu - This is for our production app. We have not changed anything in our app. Our customer already has users where we are getting webhooks. The customer added 4 new users, and then we are seeing this issue. We are not using the Call Control permission.

@Phong Vu - Is there a change in the way webhook subscription are handled? Or we need to update our app?

Was Call Control newly added for using TelephonySession Notifications ?
I see the help article is updated on 2020-07-17(Our app was graduated much before that)
https://developers.ringcentral.com/guide/voice/telephony-session-notifications
Our app had graduated much earlier on Jan-21-2020, and had been using TelephonySession notifications to get webhooks for calls without any issues until 20th Aptil 2021.

Please help us add this permission to the app as nothing has changed on our front.

The Call Control permission is required for the /telephony/session event notification since the event filter was introduced. I am no sure why your app did not catch the error earlier. Give me the app client id so I can check.