Never answer these calls, your phone is hackable just like a laptop
if you answer the call you just opened the back door to your phone.
(It's why there is no one there when you answer)
Same as you should never open a unknown Email.. and don't ever call
these numbers back.. once they get in your phone they can get in
your home wifi. Your contacts list..DON'T EVER ANSWER UNKNOWN PHONE
NUMBERS it sucks but that is the facts of the technology we use
everyday..If it's important the person will leave you a message..
You can get free phone number blockers from google play or other
app stores.