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Yes, it is a SCAM. DO NOT call their number and DO NOT pay them. If you already have given them you CC number, then contact your credit card company and ask them to reverse the charge from them.
Your Mac is virtually invulnerable to all virus out there. So you don't have to worry about that. If you have received a notice saying that your Mac is failing, corrupted etc then don't fall for it. It is a fake message. I'm not sure what you installed. But uninstall them.
Then do the following steps to clear your Safari browser. Press Command, Option and Esc keys and this will bring Force Quit window. Select Safari and click Force Quit. This will close Safari.
Then hold down Shift key on your keyboard and open Safari. Click on Safari menu and select Clear History and Website Data. On the dropdown next to Clear, select All History and click Clear History.
(If you see Reset Safari instead, click it and Reset Safari)
That will remove all traces of the message.
If you have any difficulty on following the steps above or if you are unable to remove it, please reply in here and I will continue to help you. Please let me know how it goes.
These kind of scams (phone scam, internet scam and email scam) have been there for many years and they are very deceiving. If you get those calls or messages, DO NOT take them. Just ignore those calls or messages. Remember, no legitimate company will call you about your computer issue and the fact is, how someone know your computer is malfunctioned without even having seeing your computer ?
I'm not sure what kind of applications you've installed with their help. I want you to uninstall them ASAP. If you give me the name of applications you've installed, I will help you to uninstall them. You don't need those.