You may notice that the Diva service is intermittent; furthermore and some calls over TTOld are randomly failing. On some calls, you can hear intermittent ringing, which shows that availability is random.
Please create a support ticket mentioning the numbers which are failing - both the ones used to call, and the ones being called.
<supportagent>This issue may be caused by issues related to the Diva card. The Diva card is legacy hardware, and it enters hang mode from time to time - this is not possible or easy to monitor. Restarting the DIVA Card server may resolve the issue.
Escalate the issue to the PS team including the information above.