After the first call, which I didn't pick up, I used an app to block the number that called me. Now at least once a day I get a notification that the number has called but has been blocked by my app. It's most likely a "one call" scam, where they call and let the phone ring once, just enough for a "missed call" message to pop up. If you call back, because you're curious or whatever, they'll most likely direct you to an operator and charge you up the [***] while you wait. Long distance charges, international fees...it's important to never return calls that don't leave a voicemail, unless you're made of money that these scams can just leech.