Your post, though long enough, raise few more questions or even doubts.
A rowdy bullies security guards of your company to share your mobile number. But then why he wanted your mobile number only and no one else's? The ruffian must have been sent by some mafia to create ruckus. Why he chose to come after your working hours? Why not during your working hours? Therefore, considering his behaviour, and choice of timing, the sole objective was to get your mobile number and nothing else.
Yes, Mr X who works in Admin-HR department should not have shared your contact details but that he might have done out of panic. Probably he had not faced this kind of situation and as it became unwieldy, he shared your number. However, question arises as to getting someone's mobile number is not at all difficult. Few women out of their gullibility throw all the cautions to the wind and share their e-mail ID or mobile number on social media like Facebook, Linkedin etc. However, they are not harmed for years together. Again the question arises why did the bully choose you and no one else? Therefore, there is something more than meets the eye!
Have you taken loan from some official or unofficial organisation? If yes, then have defaulted in payment of instalment? Have you incensed any local rowdy? Do you have some business that is besides your primary employment?
As far as telephone calls from the unknown numbers are concerned, what kind of threats are you getting? What kind of extortion they wanted to do? Did you record the phone calls? Why you did not approach the police?
After the incident, did security officer investigate the incident? Have your senior authorities been reported about this? Why none of the security personnel tried taking video of the bully surreptitiously? In fact this is a big security lapse as well. However, it is a secondary matter because you have written post about your personal security and not company's security.
Though you have written your post to blame the Mr X of Admin department, yet your post reveals your sense of insecurity. It is a part of your personal life. Therefore, you need to sort out that first. You may gain sympathy from your post but it is inconsequential.
From India, Bangalore