I had purchased Windows 10 Pro Product Key (OEM). When Activation failed at the first attempt, I wrote a message at the DigitalCodes.in chat window.
I soon got a call from DigitalCodes.in. After verifying my Windows Version, I was advised to get the activation done through the "Phone Activation" option provided by Microsoft. This worked.
The support staff first asked me for that Product Key. I gave the key as provided by DigitalCodes.in. This was confirmed as genuine. After this, he asked me for the Devise Code (that Win 10 had displayed on my PC) which I gave. Then he gave me the Confirmation Code that I had to enter on my PC to activate Windows 10 Pro. It worked!