Firebase SMS verification failing with error code 39

I’m experiencing an issue with Firebase where SMS verification codes are not sent to some valid phone numbers. These numbers are able to receive SMS from other providers without any issues. The problem has been observed over multiple attempts on different days. Here are the details:

{
  "insertId": "-k92yxdd8v6y",
  "jsonPayload": {
    "@type": "type.googleapis.com/google.cloud.identitytoolkit.logging.RequestLog",
    "methodName": "google.cloud.identitytoolkit.v1.AuthenticationService.SendVerificationCode",
    "status": {
      "message": "Error code: 39",
      "code": 13
    },
    "request": {
      "@type": "type.googleapis.com/google.cloud.identitytoolkit.v1.SendVerificationCodeRequest",
      "phoneNumber": "+400730679600"
    },
    "requestMetadata": {
      "callerIp": "86.124.126.97",
      "callerSuppliedUserAgent": "FirebaseAuth.iOS/10.22.0 ro.afooddeployments.afood/3.4.0 iPhone/17.4.1 hw/iPhone14_5,gzip(gfe),gzip(gfe)"
    },
    "serviceName": "identitytoolkit.googleapis.com"
  },
  "resource": {
    "type": "identitytoolkit_project",
    "labels": {
      "project_id": "ambrosia-e5ea3"
    }
  },
  "timestamp": "2024-06-17T08:49:19.981Z",
  "severity": "ERROR",
  "logName": "projects/ambrosia-e5ea3/logs/identitytoolkit.googleapis.com%2Frequests",
  "receiveTimestamp": "2024-06-17T08:49:20.859697728Z"
}

Steps to Reproduce:

1. Attempt to send an SMS verification code to the phone number
   +400730679600 using Firebase Authentication.
2. Observe that the request fails with error code 39 intermittently.

What I’ve Tried:

1. Confirmed that the phone numbers are valid and can receive SMS from
   other providers.
2. Attempted multiple times on different days to rule out temporary
   issues.
3. Reviewed Firebase documentation and error codes but found no
   specific details for error code 39.

Has anyone experienced similar issues with Firebase SMS verification? Any insights or suggestions would be greatly appreciated. Thank you!

note that +400730679600 is my own test number