Unfortunately, there is no event filter for the instant message event notification. You can easily add some code to check the text message or any other message metadata and decide to further process the message or just ignore it.

@Phong Vu
I am now subbed to this kind of event, so I can confirm that it works and you are able to subscribe to an instant message event like in the description.
My idea was to further filter the event.
For example subscribe to get notifications to messages that are of type SMS and have the subject "Hello" as in the example above:
/restapi/v1.0/account/{accountId}/extension/{extensionId}/message-store/instant?type=SMS&subject=Hello

But that failed.

@ringcentral do you guys maybe know if there's a way to handle this?