This error is encountered when the total API calls made by the applications (multiple applications) used by the client in his SF org are exceeded. The limit per SF org is completely dependent on SF editions (i.e Enterprise, Unlimited etc). 


Please note : Similar error is also encountered when the delivery report for the outgoing SMS is not getting pushed back to SF.


For example, The API calls limit for every 24 Hours for Enterprise Edition which has 20 user license, is 20,000.


Please visit the below link for detailed information regarding the same:

https://developer.salesforce.com/docs/atlas.en-us.salesforce_app_limits_cheatsheet.meta/salesforce_app_limits_cheatsheet/salesforce_app_limits_platform_api.htm


To know the distribution of the API calls made by the applications in the clients SF org, the client needs to raise a case with SF and request them to provide the necessary details.