Their next step is to convince you to give them remote access to your computer so they can fix the issue for you. Once you give them remote access, that's when the real trouble begins.
They'll start installing either fake antivirus programs that actually contain viruses, or legit A/V software that is a trial version. They may run a script (that displays on your screen) showing how many problems are on your computer. They have many tricks, but they are all geared towards you giving them your credit card information to purchase future support or to license their software.
This scam is easy to avoid. Here's how:
1. Realize that Microsoft will not call you.
2. Never give anyone you don't trust, remote access.
3. Do not buy products from callers, or give them your credit card information.
If you find that you have been scammed, uninstall any software that they installed. Run a real antivirus program to check for problems. I recommend a great free product called AVAST found here: https://www.sohonetworksolutions.com/avast
Their free for home use version provides plenty of protection for the average user.
Don't forget to maintain backups of all your files. (See my earlier post regarding Macrium Reflect Free).