Okay, after a 2-day outage, Voip.ms has reverted the fraudulent charges and my account there once again now shows a positive balance, and the ATA is able to register and place/receive calls again.

Still no mass notification to customers about the hack though, so tell everyone you know about it and get them to change all of the various passwords, disable international calling, and (for good measure) rename/delete/recreate sub-accounts.