High Volume AGW-110 - Internal Server Error. Consult RC Support

Is your app still in the sandbox? App client id?

Yes it's still there

Clien ID: ut4fLomWQSq_da_3RMn2kw

This is why you got that error.

https://developers.ringcentral.com/guide/messaging/sms/high-volume

Read the "Limitation of High Volume SMS" section.

I manually graduated your app so now you can try on your production. Remember that you have to authenticate (login) your app with the right user credentials. The one that owns the A2P SMS number.

Wow! Thank you. It seems the App has appeared in PROD

But now there is a new error ...

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What is can be?

Did you choose the production environment? Did you login as the user who owns the A2P SMS number?

Can you write your own code or copy paste sample code from the dev guide to send test messages?