curl --request DELETE \
  --url https://rest-api.smsmngr.com/v1/optout \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '
{
  "to": "420777123456"
}
'

{
  "success": true,
  "optout": {
    "removed": 1,
    "remaining": 5,
    "invalid": []
  }
}