before you start searching for a DTMF issue (DTMF referring to the key presses of the IVR), 
you should really rule out that no change has happened on the admin portal. 

Here are some things you can do:
-in the admin portal > More > Audit Trail > see if anyone has made any changes to the IVRs, focusing on the time when the issue first occurred

-use different devices to call your IVR menu. actual DTMF issues are usually carrier related. For example, is the issue present only with external mobile phones, or also landlines? are only calls from a specific provider affected or all calls? Are internal calls affected - for example if you log into the RingCentral web app and dial the IVR menu are you able to navigate it (try clicking on the pop up dial pad with the mouse if using the keyboard is not working)? - if the IVR is not working even internally I think it is more likely to be a change on the admin portal that caused the issue.

Most importantly whenever you are doing a test call to troubleshoot this, note down what happened, so you can provide it to RingCentral support if they need call examples. Like this:

From:
To:
Date & time: 
what happened on the call:
 

Hello @Yashin R,

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