A number of people have been running into this so I want to bring attention to a workaround that @ptsv has found:
Hi,
I managed to register for the free account.
What I had to do was to verify my seemingly invalid number through text/call and then I had the green light to proceed with my registration.
It seems that in the backend it reads the number correctly and this is just a visual issue.
Hopefully this will help others who may be running into the same issue. Thank you again to @ptsv for sharing this!
If you are still running into issues, try the solution to this similar issue:
Problems with UK phone number when attempting to register for an Azure account