Error: Your org may be locked out for external API calls.

Full Error Message:
Opero Fax App could not register your Salesforce Org.
Your org may be locked out for external API calls.
Contact Support for further help. Attempting to register the following Org ID: {Org Id}

Solution:
Customer who receives this error by email should contact Salesforce support (or their Account Executive) to have them unlock external API calls. Also ensure that API is enabled in Salesforce org and has not reached the daily API call limit.