A customer has an active session in your system and when they call back during that active session and choose the 2nd option to "start a new parking session on a different vehicle", they hear a message stating they'll be transferred to your helpdesk. However, they hear music and then an error message stating that their call can't be transferred and it disconnects.
It used to work as designed but now suddenly, this happens on any of your IVR numbers. You've tested and reproduced the issue yourself.
The issue might be happening due to the reason that the main helpdesk might have been deleted and not added to the new route list. In order to solve this issue, the main helpdesk number will have to be added to the new route list.
Raise a support ticket explaining your issue and the Voltdelta support team will be happy to help you.
In a similar case, Verizon made changes without notifying anyone. They are not doing the HD transfer directly and instead transferring the call to another carrier to make the outbound transfer. The customer's main helpdesk number will have to be provided to Verizon to be added to the new route. Create a Jira and escalate the issue to the PS team to resolve the customer's problem.