You need to retrieve the Phone Numbers on the extensions . Check the following reference: : https://developers.ringcentral.com/api-reference/

One way of doing is calling the create SMS API from your code using a scheduler . If you are using Java, you can use a Task scheduler or timer to call the API periodically. It is also possible in c# I believe.

When can we have this group SMS scheduler on our RingCentral Application because our customer is requesting for it.

It seems what you are asking for is a feature from the RingCentral app. I will move this question to our customers support so they can take a look at this. Even I know that this is not doable with any current RC app.