so I’m using Firebase Phone Auth in my production web app and I am experiencing some issues with some phone numbers. The issue mainly occurs with phone numbers from

• Slowenia (+386)
• Hungary (+36)

and other eastern European countries, however, it does not happen on every phone number from these countries – some registration have gone through.

Trying to trigger the OTP, after completing the reCaptcha, accounts:sendVerificationCode throws this error:

[
  {
    "message": "Error code: 39",
    "domain": "global",
    "reason": "backendError"
  }
]

I am not hitting any quota and have not black or whitelisted any country.

Does anybody have similar experience and could help me out on how to tackle them?