I am trying to add my Estonian (+372) phone number to complete my contributor profile, but I get a persistent error message

Here’s what Google has to say:

“Estonia is an EU member state where Adobe officially operates and supports standard account services and contributor registrations. If you are experiencing a phone verification error (such as "An error occurred while performing this action. Please try again later") using an Estonian phone number (+372), it is typically caused by a temporary SMS gateway glitch, browser cache issue, or a network-side problem rather than a regional ban.”

So maybe try again later or possibly clear your cache.