Unable to connect to bot by using sandbox

I have configured IVR channel and enabled Sandbox for testing purpose.
When I dial the generated number then it asks for Pin and secret code.
After Entering Secret code, there is silence on the call for few seconds and after that call is getting disconnected after playing ‘Serious Error has occurred, Exiting’.

Hi @prajapatimits,

Kindly let us know if you have a dialog task that has a valid dialog flow.
Or let us know if you have a welcome event configured.

Yoga Ramya