Why Spoof Caller ID?
When it comes to Social Engineering, being able to spoof your caller ID is extremely important so as to not raise any form of suspicion. By using the exact number over a certain period of time the company will definitely blacklist your number as not trustworthy and you will not be able to SE.
By spoofing your Caller ID, the number the customer operation gets will be a completely brand new number that to them will seem like a new customer who does not have a history of attempted fraud. Thus raising your odds of a successful SE.
And yes, the method is free for you to use and you can spoof unlimited amounts of Caller ID. Highly recommended if your SE requires you to talk to customer service.