Bulk SMS for Sri Lanka.
In their language.

# one request, any number of recipients
curl -X POST https://avcsms.lk/api/v1/sms/send \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"to": "0771234567", "message": "Your OTP is 482913", "sender_id": "MYSHOP"}'

$ch = curl_init('https://avcsms.lk/api/v1/sms/send');
curl_setopt_array($ch, [
  CURLOPT_POST => true, CURLOPT_RETURNTRANSFER => true,
  CURLOPT_HTTPHEADER => ['Authorization: Bearer YOUR_API_KEY', 'Content-Type: application/json'],
  CURLOPT_POSTFIELDS => json_encode(['to' => '0771234567', 'message' => 'Your OTP is 482913']),
]);
$reply = json_decode(curl_exec($ch), true);

const res = await fetch("https://avcsms.lk/api/v1/sms/send", {
  method: "POST",
  headers: { Authorization: "Bearer YOUR_API_KEY", "Content-Type": "application/json" },
  body: JSON.stringify({ to: "0771234567", message: "Your OTP is 482913" }),
});
console.log(await res.json());

import requests

r = requests.post("https://avcsms.lk/api/v1/sms/send",
    headers={"Authorization": "Bearer YOUR_API_KEY"},
    json={"to": ["0771234567", "0712345678"], "message": "ඔබේ OTP 482913"})
print(r.status_code, r.json())