How Do I perform a flash call verification through rest API?

Question

I have a list of over 1000 phone numbers that I am trying to verify using the sinch verification REST API by placing a flash call. However, I always get this response from the endpoint...

[
    callId:fa61c8ae-2936-4c3d-a89c-ca852d9bd464, 
    reason:Expired, 
    method:flashcall, 
    identity:[
        endpoint:+2348166180681, 
        verified:false, type:number
    ], 
    id:fa61c8ae-2936-4c3d-a89c-ca852d9bd464, 
    event:VerificationResultEvent, 
    status:FAIL
]

I am carrying out the following steps...

1. Make a POST request to https://verificationapi-v1.sinch.com/verification/v1/verifications with the following body...

   ["identity" : ["type": "number","endpoint": "+xxxxxxxxxx"], "flashCallOptions": ["dialTimeout": 10],"method": "flashCall"]

2. Listen to a verificationRequestEvent and respond with [action: "allow"]

When I do this, I get a ping to my verification webhook with the VerificationResultEvent shown above.

Is there something I am not doing right? My current suspicion is that instead of just responding in step two with [action: "allow"], I am supposed to make another POST request?