Don't answer it.
Its a scam, Microsoft does not call anybody to provide technical support for Windows 10:
Warning: There are "Tech Support Scams" pretending to be Windows 10 Support. Please be vigilant. Microsoft never calls users to inform them their license is expiring or troubleshoot problems. Neither should you get an email. Learn more in the following article:
PSA: Beware of Windows 10 Activation Tech Support Scams
http://www.groovypost.com/news/beware-of-window...
Report it:
https://www.microsoft.com/reportascam/
Tech support scams - Windows Defender Security ... - Microsoft
https://www.microsoft.com/en-us/wdsi/threats/su...
Protect yourself from tech support scams - Microsoft Support