hey i have this same problem with this number.. i think it is cause nimbuzz. i download it and then after that this number is calling me sometimes even at night, it make me angry cause one day it woke me up at 4 am..
hey it's again me. i tried to fing out who is this and i know the answer now. the people who has nimbuzz on te phone, have to type their phone number to get "buzz". if this number +31313131 is calling, that means somebody send to u "buzz" to open nimbuzz when u're offline. i will be happy if i helped someone with that problem :)