It is 100% secure IF YOU INITIATE THE CALL TO MICROSOFT OFFICIAL phone number and they re-direct you call to whoever.
Never secure if YOU RECEIVE the call first.
That depends on how you define security. Nothing digital is secure. Apparently you have not read the numerous threads on FR or read Snowden’s comments regarding government cell phone surveillance.
Whether it be cell phones, computers, home security systems, it does not matter as all of it can be hacked. The latest craze in security is a VPN. By now there are those that have figured out a way around it. Once that becomes obvious there will be another security tool to be sold.