Who called from +30 21 0682 0003?

Another call from this number today, which shows as a call received but blocked. Yesterday, a couple of hours after the original call from this number, which I had blocked by that time, I got another call labelled as unknown number while it was ringing, which shows in call log as "Number is null".

This number called my mobile phone. When I answered, there was immediately what sounded like a dialling tone, similar to when your call is being forwarded to another number, then the call was cut off. I can't think of any reasons anyone would do this, other than to check if a mobile phone number is active or to do a voice recording of the person saying hello. Perhaps someone is using an automated system to check if mobile numbers are active to call again later, either from the same or a different number. I find this type of call quite sinister anyway, so I have blocked the number on my phone anyway.