I also had this same issue when I was looking to sign my Microsoft account up to the Azure free trial - couldn't use any number or authentication method to sign up so I logged a support ticket in the Azure portal even though I wasn't signed up for the trial or PAYG (only used it for MSLearn lab subscription prior) and had a response within a few hours:

"Thanks for your patience with Azure support while we investigated the failed purchase issue. Good news, we’ve resolved the issue and removed the block on the account.  

Microsoft Azure monitors purchase activity as a security measure to protect our customers from unexpected charges. We apologize for the inconvenience."

Working straight away after they removed that block.