Visualforce TwiML page and Apex REST method for transcribing a voice message in Twilio and saving is as an Activity to a Lead.

MessageToTask.cls

@RestResource(urlMapping='/messagetotask')
global class MessageToTask {
    @HttpPost
    global static void incomingMessage() {
        String expectedSignature = 
            RestContext.request.headers.get('X-Twilio-Signature');
        String url = 'https://' + RestContext.request.headers.get('Host') + 
            '/services/apexrest' + RestContext.request.requestURI;
        Map <String, String> params = RestContext.request.params;

        // Validate signature
        if (!TwilioAPI.getDefaultClient().validateRequest(expectedSignature, url, params)) {
            RestContext.response.statusCode = 403;
            RestContext.response.responseBody = Blob.valueOf('Failure! Rcvd '+expectedSignature+'\nURL '+url/*+'\nHeaders'+RestContext.request.headers*/);
            return;
        }
        
        // Twilio likes to see something in the response body, otherwise it reports
        // a 502 error in https://www.twilio.com/user/account/log/notifications
        RestContext.response.responseBody = Blob.valueOf('ok');
        
        // Extract useful fields from the incoming transcription
        String leadNumber     = params.get('Caller');
        String campaignNumber = params.get('To');
        String transcription  = params.get('TranscriptionText');
        
        // Look for a matching lead
        List<Lead> leads = [SELECT Id FROM Lead WHERE Phone = :leadNumber LIMIT 1];
        Lead lead;
        if (leads.size() > 0) {
            // We found a match!
            lead = leads[0];
        } else {
            // Create and insert a new Lead
            lead = new Lead(LastName = 'From Transcription',
                Company = 'From Transcription',
                Phone = leadNumber);
            
            try {
                insert lead;
            } catch (DmlException dmle) {
                reply(campaignNumber, leadNumber, 'An error occurred. Sorry.');
                return;
            }
        }
        
        // Create and insert a new Task
        Task task = new Task(
            ActivityDate = Date.today(),
            CallDisposition = 'Lead left a message',
            CallType = 'Inbound',
            Description = transcription + 
                '\n\nMessage recording at https://api.twilio.com/2010-04-01/Accounts/'+params.get('AccountSid')+'/Recordings/'+params.get('RecordingSid'),
            Status = 'Completed',
            Subject = 'Phone Message from '+leadNumber,
            Type = 'Call',
            WhoId = lead.Id
        );
        insert task;
    }
}

MessageToTask.page

<?xml version="1.0" encoding="UTF-8" ?>
<apex:page sidebar="false" showHeader="false" contentType="application/xml">
  <Response>
    <Say>Hello! Please leave a message.</Say>
    <Record maxLength="120" transcribe="true" transcribeCallback="/services/apexrest/messagetotask"/>
  </Response> 
</apex:page>

How do i test this class and page
Do i need to open this vf page in mobile.

Please let me know how to test this