Can't add phone number to account

Been trying to redeem Microsoft Points for a few months but come up against the same issue. It asks me to add a phone number to my account.

I follow the steps to do so, the code is sent to my phone and I input it into the website and get this error:

There's a temporary problem with the service. Please try again. If you continue to get this message, try again later.

I've been trying to do this for a few months. In that period I've attempted on different devices, operating systems, browsers and connections to rule out caching and DNS and anything I'm presuming a Microsoft agent will blame for this issue.

:)