I am using ringcentral-csharp. I have a very simple send SMS and initiate a RingOut methods that has worked fine with the sandbox. I got my production credentials and approval. When I switched to production the RingOut works fine (so the key and secret are good) but when I try to send an SMS I get the following error: RingCentral.Http.ApiException: The requested feature is not available.
For the from I started with +18667268442 then I tried it without the + as 18667268442 with the same result. The extension is 4. Some time a few years back someone at customer service changed the main extension to 4 from whatever it was. I don't remember what we were fixing but it has been like that for a long time.
I am able to use the Ringcentral desktop and mobile app with this combination.
The problem is that you cannot send SMS from toll-free number. Such messages may be not routed properly through carrier networks. Especially replies to these messages. So you need local number to specify it as "from".