Delaying Tactics/Avoiding Providing OTAC's
Criminals may delay answering questions if they need more time to find the information you are requiring, whereas a customer would not delay.
Long Delay At The Start Of The Call/Distored Audio
This is often a sign you are on the phone with a criminal as the line could be connecting to someone in a call centre, and the connection may be bad if the call centre is based overseas.
Excuses As To Why OTAC Can't Be Passed
This indicates the caller is not receiving the OTAC directly and is sourcing this separately from the customer.
Inconsistent Answers/Avoiding Simple Questions
Inconsistent answers could be a sign that the person on the other end is impersonating your customer as they may make mistakes on the questions you ask.