The next step is to see an actual invite from the provider. Navigate to your Dashboard / Activity Log / Settings and set the logging level to verbose.
Then make a call to the system and wait for it to fail. Then go back into your Activity log, click the filter button and select the last call in the "Search by call" drop down menu.
Scroll all the way to the bottom and find the first Invite message you see. Look in the "To:" header and check how the number is formatted. If you are having trouble send me a d.m. with a screenshot and I will have a look.