add support for tls with srtp media encryption (#83)

lib/call-session.js

@@ -256,7 +256,8 @@ class CallSession extends Emitter {
               const obj = {
                 name: vc.name,
                 diversion: vc.diversion,
-                hostport
+                hostport,
+                protocol: o.protocol
               };
               if (vc.register_username && vc.register_password) {
                 obj.auth = {
@@ -266,6 +267,16 @@ class CallSession extends Emitter {
               }
               mapGateways.set(u, obj);
               uris.push(u);
+              if (o.protocol === 'tls/srtp') {
+                /**  TODO: this is a bit of a hack in the sense that we are not
+                 * supporting a scenario where you have a carrier with several outbound
+                 * gateways, some requiring encrypted media and some not.  This should be rectified
+                 * but it will require more significant changes and right now it seems
+                 * like a rare use case -- encryption is usually an all or nothing requirement.
+                 */
+                this.logger.info({u}, 'using SRTP for outbound call');
+                this.rtpEngineOpts = makeRtpEngineOpts(this.req, false, true, true);
+              }
             });
             this.logger.debug({uris, voip_carrier_sid}, 'selected outbound gateways for requested carrier');
           }