Number keeps calling and leaving no message

just got a missed call from this number. i called back and it says this is a non-working number. how are these people getting my number? it's a straight talk phone, i just got it and only 3 people have the number.. 2 family members and 1 friend.

when I answered call there was no one there.  I tried to call back and would ring maybe 4 rings and then cut off.