We have the same issue. Other things that have changed that have changed for us:

The video button is greyed out so you can't switch a call to a video call.
A red 'record' button has appeared on the dialler.

Are you seeing these too?

We've got the same problem this morning on the Windows application as well. The Transfer and Add Call buttons are both greyed out on every call, as is the transfer to video button.

Like you, we've also got a red call recording indicator on the dialler now.

RingCentral was issued an update this morning which looks to have broken these, as these features are all still available to use in the Android mobile app.

The latest iOS app also breaks this in a different way, you can add a call to start the creating a conference, but the option to merge them together is greyed out.

My Android app has an update pending, so presumably if I let that install it will also break these features.

RC have rolled back the update for us, so presumably they have for everyone else. You need to manually use the 'Restart to update' button in Help > About to get the older version to install, then wait a few mins and the version number should revert to the below and fix the problem.

Version: 21.3.35.664 Web, 21.3.31.1932 Windows

Date: 2021-09-29 08:59:00

That works for me