Send SMS via RingCentral API by using different numbers and extensions

I have a problem of sending SMS via API within one account where we have a bunch of numbers with their own extensions. The goal is to be able to use any of these numbers for sending SMS from Salesforce via callout. I've created an app with "SMS", "Read Account" permissions. When I try to send a request from Postman using "Account" credentials with or without extension number, I receive an error: "Phone number doesn't belong to extension". I cannot log in with default account credentials and different extensions. I do not have other users credentials. I cannot add number to extension as it already have own extension. Is there any way I can solve this? Please, advice.