The reason for the mobile number under (object Name) object being empty when clicked on the Send SMS Button is - check if a custom field named "ID" is present on the (object Name) object. If so, it will result in a conflict with the "record id" which gets picked when we click on the Send SMS button.
We need to change the API name of the custom field "ID" - from "Id__c" to "IDS__C". Thus, resolving the conflict. Please refer the below screenshot for the same:
After doing the above-said, you can try to send out a single SMS from the (object name) object and the Send SMS button should work as expected.