There is some cases depends on selected mode order status remains unverified:
Call mode :
If customer don’t receive the call
Customer number not exist
Network busy
Not in network
Customer receive the call but don’t press any input or press wrong input
Failed to call from IVR service
SMS Mode :
SMS not delivered
Phone number not exist
Not in network
SMS delivered but customer don’t open the confirmation link
OTP Mode :
OTP SMS does not sent due to invalid number or network issue
OTP received but customer didn't add correct number
Added invalid OTP
In these cases if a customer fails to confirm their order then order status will be pending (unverified).