safari suddenly froze and message said to call number at microsoft to avoid Trojan virus

I was using Safari on my mom's Mac today and went to You Tube. suddenly a message appeared from http://tubeblog1.com saying that the system was infected with Trojan...it went on to explain what that was and said to call 1-844-813-8227 immediately to resolve problem. I was eventually able to unfreeze Safari and everything seems fine. However, about 3 hours later a man called my mother's land line from India (accent was heavy so I assumed it was from there!) and asked if the computer needed to be fixed!!!!!! HOW DID HE GET MY MOTHER'S UNLISTED PHONE NUMBER? We never called that number and it seems weird that he new her number was somehow connected to her Mac? Like I said, she has an unlisted number. Please help...this is really weird and SCARY!!!! I confronted the guy on the phone and he hung up on me. How can i make sure there is no malware or virus in her computer now?