Hi i had a missed call from a similar 4 digit number,happenend a couple of time while in work and i missed the call.After a few weeks of wondering who or what it was i tried to return the call but got an engaged tone.Then luckily for me the number rang again and i answered it.Turned out it had been my sister calling me from work where only an extension number shows up with no prefix or any area code,knowing this i suggest you ask your friends who who for a large organisation if this could be their extension number,hope this helps,this was only the case for me i dont want a response saying you rang back and it cost you 100 euro for some scam!!!!Best of luck