server.js

require('dotenv').config();
const app = require('express')();
const bodyParser = require('body-parser');

app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));

app
    .route('/webhooks/inbound-sms')
    .get(handleInboundSms)
    .post(handleInboundSms);

const concat_sms = []; // Array of message objects

const handleInboundSms = (request, response) => {
    const params = Object.assign(request.query, request.body);
    console.dir(params)
    if (params['concat'] == 'true') {
        /* This is a concatenated message. Add it to an array
           so that we can process it later. */
        concat_sms.push({
            ref: params['concat-ref'],
            part: params['concat-part'],
            from: params.msisdn,
            message: params.text
        });

        /* Do we have all the message parts yet? They might
           not arrive consecutively. */
        const parts_for_ref = concat_sms.filter(part => part.ref == params['concat-ref']);

        // Is this the last message part for this reference?
        if (parts_for_ref.length == params['concat-total']) {
            console.dir(parts_for_ref);
            processConcatSms(parts_for_ref);
        }
    } else {
        // Not a concatenated message, so just display it
        displaySms(params.msisdn, params.text);
    }

    // Send OK status
    response.status(204).send();
}

const processConcatSms = (all_parts) => {

    // Sort the message parts
    all_parts.sort((a, b) => a.part - b.part);

    // Reassemble the message from the parts
    let concat_message = '';
    for (i = 0; i < all_parts.length; i++) {
        concat_message += all_parts[i].message;
    }

    displaySms(all_parts[0].from, concat_message);
}

const displaySms = (msisdn, text) => {
    console.log('FROM: ' + msisdn);
    console.log('MESSAGE: ' + text);
    console.log('---');
}

app.listen(process.env.PORT);