That's not very much information to go on. My best guess is there's a problem with the callback number you are using. Can you show me what you have that set as?
Sending me the flow likely won't help, unless your error is caused by an action after the Create Callback action (creating a callback is a silent action, so you could be hitting an error in a later action). Contacting support so they can dig into the logs to see exactly what happened is the only way to find out what the problem is. They'll need the conversation id of the failed call.