The number you have dialled is temporality unavailable

Simple test, can you dial that number using your phone?

@Catherine Kirkland What @Phong Vu mean to say that have you tried dialing the same number with your personal phone or any other phone for testing? If yes, are you getting the same message that you got from RingCentral?

I still don't get what you do and what is going on. "calling form my iPhone and through the RingCentral website". What is "through the RingCentral website"? How do you call a website?

It's so hard to understand your original question. These questions are for you to answer

1. "I have set up an app that calls a web hook on an incoming call and managed to do soon initial testing fine by calling the main number". Explain this . Is this because of your app, you could call the main company number?

2. "I have moved the app to the clients account and have it in the sandbox". How? how did you move an app from your account to your client account?

3. "but when I call the main number (sandbox) I get a voice response "The number you have dialled is temporality unavailable" and that is all I get." Is this something related to your app? Without your app involved in this, can your client make a call to any number in its sandbox account?

I could say that your app has nothing to do with the problem where you cannot dial a number even from your mobile phone. It is just your customer's phone number/phone setup.

I recommend you open a support ticket so the developer support team can set up a call to walk you thru.