curl --request POST \
  --url https://example.com/callback \
  --header 'Content-Type: application/json' \
  --data '
{
  "command": "initiate",
  "msisdn": "255762089337",
  "session_id": "4574",
  "operator": "vodacom",
  "payload": {
    "request_id": 0,
    "response": 0
  }
}
'

{
  "msisdn": "2556730893370",
  "operator": "vodacom",
  "session_id": "33545",
  "command": "continue",
  "payload": {
    "request_id": "1",
    "request": "enter amount"
  }
}