No more numbers available for new operations time window set

What causes this issue?

All valid internal numbers have been assigned for the operations time
window sets.

System Response

The operations time window sets cannot be created; the system cancels
all subsequent processing.

How to fix this error?

The internal number range must be extended before more operations time
window sets can be entered into the system. Please contact your system
administrator.

Error message extract from SAP system. Copyright SAP SE.