If a phone line is busy when Verify is trying to ring a number, Verify will exit the workflow and set it as failed. You will not be charged for these requests. You can submit another request to Verify this number (however we'd recommend you wait some time before doing so).
Nexmo Verify will ring the number for maximum 30 seconds. If the end user does not pick up, the verification workflow will terminate and the status of the verification request will be FAILED.