One API. Your phone sends it.

curl -X POST "$GATEWAY_URL?action=enqueue" \
  -H "X-API-Key: $GATEWAY_KEY" \
  -H "Content-Type: application/json" \
  -d '{"channel":"sms","to":"+237600000000",
       "body":"Your code is 481 922","idem_key":"otp-1042"}'
# {"status":"ok","id":123,"duplicate":false}

# later: what happened to it?
curl "$GATEWAY_URL?action=message&id=123" \
  -H "X-API-Key: $GATEWAY_KEY"
# {"status":"ok","message":{"id":123,"status":"sent", ...}}

$ch = curl_init(getenv('GATEWAY_URL') . '?action=enqueue');
curl_setopt_array($ch, [
    CURLOPT_POST           => true,
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HTTPHEADER     => [
        'Content-Type: application/json',
        'X-API-Key: ' . getenv('GATEWAY_KEY'),
    ],
    CURLOPT_POSTFIELDS     => json_encode([
        'channel'  => 'whatsapp',
        'to'       => '+237600000000',
        'body'     => 'Your repair is ready for pickup.',
        'ref'      => 'repair-42',
        'idem_key' => 'repair-42-ready',
    ]),
]);
$reply = json_decode(curl_exec($ch), true);   // ['status' => 'ok', 'id' => 124]

const res = await fetch(`${GATEWAY_URL}?action=enqueue`, {
  method: 'POST',
  headers: { 'Content-Type': 'application/json', 'X-API-Key': GATEWAY_KEY },
  body: JSON.stringify({
    channel: 'ussd',
    body: '*126#',
    ussd_steps: ['1', '699999999', '5000'],   // typed into the menu, in order
    sim: 0,
    ref: 'momo-77',
  }),
});
const { id } = await res.json();

// poll for the operator's answer
const r = await fetch(`${GATEWAY_URL}?action=message&id=${id}`, { headers: { 'X-API-Key': GATEWAY_KEY } });
const { message } = await r.json();   // message.status, message.result, message.error