curl --request POST \
  --url https://rest-api.smsmngr.com/v1/messaging/validate \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '
{
  "to": [
    {
      "phone_number": "420777123456"
    }
  ],
  "body": "Hello!"
}
'

{
  "success": false,
  "validate": {
    "accepted": [
      {
        "key": "0",
        "phone_number": "420777123456"
      }
    ],
    "rejected": [
      {
        "key": "1",
        "phone_number": "123"
      }
    ],
    "errors": [
      {
        "key": "1",
        "field": "to[1].phone_number",
        "message": "Phone number is not in a valid format",
        "code": "302"
      }
    ]
  }
}