That's because they never had your number. These scammers call random number combinations until someone answers. Unfortunately, now that you answered they know you are a real person, and you have just inadvertently added your own number to their scam calling list. They use a computer to fool your caller ID into saying an American phone number, but in actuality they are located in Nigeria or another 3rd world country. I never answer numbers I don't know. I'll Google the number (90% of the time it's phone scammers, phishers trying to steal your information, so BEWARE!) If it's important or legitimate they will leave a message or call back!! I hope this helps people protect themselves from these asshats!!!!
Just don't answer unknown numbers!! They call random numbers and once you answer they know that your a real person. Now they have your number and know your real person they will continue to call you! Important or legit callers will leave a message!! Google unknown numbers and you will find these pages and feel better knowing they didn't get you!! Never Answer These calls, or their harrassment of you will increase tenfold!