A Twilio app (via a series of Functions) that sends the caller to voicemail, but only if they correctly answer a captcha

twilio-main.js

const record = (twil) => {
    twil.record({
        transcribe: true,
        transcribeCallback: "/transcribed", // see twilio-vm.js
        action: "/call-missed", // see twilio-missed.js
        maxLength: 90,
        finishOnKey: "*"
    })
}

const challenge = (evt) => {
    const rand = (max) => Math.floor(Math.random() * max)
    const first = rand(9)
    const second = rand(9)
    const answer = Math.abs(first - second)
        
    return {
        gatherOpts: {
            numDigits: 1,
            action: `/main?expectedAnswer=${encodeURIComponent(answer)}`
        },
        challenge: `Hello caller from ${evt.FromCity}. To leave a message, first use your keypad to enter the absolute difference between ${first} and ${second}.`
    }
}

exports.handler = function(context, event, callback) {
    const voice = new Twilio.twiml.VoiceResponse()
  
    if (event.expectedAnswer === undefined) {
        const ch = challenge(event)
        voice.gather(ch.gatherOpts).say(ch.challenge)
    } else if (event.expectedAnswer === event.Digits) {
        voice.say("Thank you human, please leave a message after the tone.")
        record(voice)
    } else {
        voice.say(`The answer was ${event.expectedAnswer}. Goodbye.`)
        voice.hangup()
    }
  
    callback(null, voice)
}

twilio-missed.js

exports.handler = function(context, event, callback) {
  context.getTwilioClient().messages.create({
    to: process.env.TO,
    from: process.env.FROM,
    body: `${event.RecordingDuration}s call from ${event.From} (${event.FromCity}, ${event.FromState} ${event.FromZip})`
  }).then(msg => {
    callback(null, msg.sid)
  }).catch(err => callback(err))
}

twilio-vm.js

exports.handler = function(context, event, callback) {
    if (event.TranscriptionStatus !== "failed" && event.TranscriptionText) {
        context.getTwilioClient().messages.create({
            to: process.env.TO,
            from: process.env.FROM,
            body: `${event.From}\n${event.TranscriptionText}\n${event.RecordingUrl}`
        }).then(msg => {
            callback(null, msg.sid)
        }).catch(err => callback(err))
    } else {
        callback(null, null)
    }
}