Twilio function to forward calls to a SIP host that uses SRV (RFC 3263) records for load balancing. Twilio doesn't natively support SRV for looking up SIP servers. So, calls made to most SIP hosts result in a DNS failure. This script is a workaround for that issue. https://stackoverflow.com/questions/48160769

Twilio-function-SRV-forward.js

const dns = require('dns');

let sipUri = '[email protected]'; // Any sip uri using srv records should work
let protocol = 'udp';
let region = 'us2' ;

exports.handler = function(context, event, callback) {    
  var user =   sipUri.split('@')[0];  
  var host =   sipUri.split('@')[1];  

  // generate the TwiML to tell Twilio how to forward this call
  let twiml = new Twilio.twiml.VoiceResponse();
  const dial = twiml.dial({
    answerOnBridge: 'true' // Don't answer incoming call until outgoing call is answered
  });
  dns.resolveSrv('_sip._'+protocol+'.'+host, (err, addresses) => { // Get SRV records for host
    var resolvedhost = addresses[0].name+':'+addresses[0].port; // Fisrt SRV record returned
    dial.sip('sip:'+user+'@'+resolvedhost+';region='+region); 
    console.log(twiml.toString());
    // return the TwiML
    callback(null, twiml);
  });
};