Who called from 31507

Today i had a missed call from number +31507. For me it's a weird number because here in the Netherelands (+31 is our country number) we are using a 10 digit number for phonecalls. I don't know if it's save to call back or to answer the phone.

who the hell is calling me from + 31507

I received 3 missed call from +31966 last night, who are they ?

How are these people i have no idea time of call 16.06pm 1St July 2012