How, where, and is it possible to check if I really have any bitcoins blocked in my name? (If yes, then how)
The answer is simple: Bitcoins aren't stored "in any name". All you need to access your Bitcoins is the private key. There's no names, no paperwork and no intermediary involved.
give them 5% so that they can transfer me the bitcoins
The modern version of the Nigerian scam. As a rule of thumb: if you need to pay before you get paid, you're getting scammed.
A few years ago, I was defrauded by some company and they fooled me into investing money with their fake company.
Chances are your number is on a high-value list of targets that have been scammed before. You could consider changing phone numbers to get rid of it.