Email, SMS and WhatsApp from Next.js,
in one server-side function.

# .env.local (gitignored by create-next-app)
# No NEXT_PUBLIC_ prefix: the key stays on the server
UM_API_KEY=um_live_xxxx
# Only for the internal route handler of step 4
INTERNAL_SECRET=change-me-to-a-long-random-string

// lib/notify.ts
import 'server-only'; // the build fails if a client component imports this file

export type Channel = 'EMAIL' | 'WHATSAPP' | 'SMS';

export async function notify(input: {
  email?: string;
  phone?: string; // E.164, e.g. +33612345678
  channels: Channel[];
  subject: string;
  text: string;
}) {
  // Drop the channels that have no destination
  const channels = input.channels
    .filter((c) => (c === 'EMAIL' ? input.email : input.phone))
    .map((channel) => ({ channel }));
  if (channels.length === 0) return null;

  const res = await fetch('https://api.unlimitedmessaging.app/message', {
    method: 'POST',
    headers: {
      Authorization: `Bearer ${process.env.UM_API_KEY}`,
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      to: { email: input.email, phone: input.phone },
      channels,
      subject: input.subject,
      text: input.text,
      priority: 'transactional',
    }),
  });
  if (!res.ok) throw new Error(`Unlimited Messaging ${res.status}: ${await res.text()}`);
  return res.json(); // { dispatchId, messages: [...] }
}

// app/signup/actions.ts
'use server';

import { after } from 'next/server';
import { notify } from '@/lib/notify';

export async function signUp(formData: FormData) {
  const email = String(formData.get('email'));
  const phone = String(formData.get('phone') ?? '') || undefined;
  // ...validate the input, rate limit, create the user

  // Runs after the response is sent: the form never waits for the message
  after(() =>
    notify({
      email,
      phone,
      channels: ['EMAIL', 'WHATSAPP'],
      subject: 'Welcome to Acme',
      text: 'Your account is ready. Reply to this message if you need help.',
    }).catch(console.error),
  );
}

// app/api/notify/route.ts
import { notify } from '@/lib/notify';

export async function POST(req: Request) {
  // For your own jobs and services only, never a public endpoint
  if (req.headers.get('authorization') !== `Bearer ${process.env.INTERNAL_SECRET}`) {
    return new Response('Unauthorized', { status: 401 });
  }
  const { email, phone, subject, text } = await req.json();
  const result = await notify({ email, phone, channels: ['EMAIL', 'SMS'], subject, text });
  return Response.json(result);
}

curl -X POST http://localhost:3000/api/notify \
  -H "Authorization: Bearer $INTERNAL_SECRET" \
  -H "Content-Type: application/json" \
  -d '{"email":"[email protected]","phone":"+33612345678",
       "subject":"Order #1042 shipped","text":"Your order #1042 is on its way."}'