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Why not pick up the phone the next time that number calls? I think there is a difference between receiving international calls and making international calls but you could check this with your cell phone carrier first. I could be wrong.
I had another thought, can you look on his social media and see if he lists a phone number? Could you message him through his social media and simply ask him if he has been calling you? No need to explain or write anything long, just ask the one question. This would be driving me nuts, but I am overly curious person.
Your other option is to simply block this number so you stop getting the calls. Mostly, I also think we believe what we want to believe. So this could be legitimate or it could be a scammer and not leaving a message or emailing you or contacting you via your own social media makes me think this is not legitimate. You could try an app that limits scammers, nomorobo is one. If this is a scammer, the app will stop the call as it is ringing. Good luck on this. And I wish you the best for your surgery.