Someone keeps calling me with a blocked number?

Since a week ago, someone that my I phone refers to as 'blocked' keeps calling me. They don't talk, I just hear rattling and other strange noises then the call ends after about 5 seconds. (I think half way through a voice mail mentioned below I heard a toilet flushing or something similar) just this afternoon when I'd totally forgotten about it; they must have left a minute or so long voice mail of random noises and rattling. As far as I know there are no blocked numbers on my phone, if they were telesales or something; surely they would speak at one point. I look through my calls list and see the name 'blocked' there twice (they called twice).in the voice mail I struggled to make out any noises, it was just the usual rattling; they phoned me twice within 5 minutes and left it for almost a week. I'm quite cautious as to who I give my number to and for them to appear as blocked, wouldn't they need to actively hide their number? I can't reply to them either. Is there any way I can record the calls and show them to people who might be able to help? Can I unblock the number and see it? I don't want to stop them, but catch them in the act and find out who they are. I'm using an iphone 4 on IOS 6.1, o2UK network. I'll pass on services to unblock the number; but I here there are things you can do to reveal it for free or with a trial. I know it might just be my mates messing about or someone with a broken mic that doesn't realize it (doesn't explain blocked) but surely I can find out who it is.

And why do you use technology… Does technology pay you? I know your saying in your -stupid- dum azz-- . But really u lost all respect from me

Thats kind of creepy unless it gets really serious have an expert trace the calls and have the # blocked. In my opinion this is probably someone trying to spook you or a broken mic.lol. For now just don't answer

Somebody fat fingered your number by mistake. Don't even bother to answer it anymore - Let it ring itself until someone checks it.