diff --git a/.gitignore b/.gitignore
index 3c3629e..1bd7226 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 node_modules
+*.swp
diff --git a/sip.js b/sip.js
index 5f1ff94..86e9dbf 100644
--- a/sip.js
+++ b/sip.js
@@ -769,7 +769,13 @@ function makeUdpTransport(options, callback) {
           msg.headers.via[0].params.rport = rinfo.port;
       }
 
-      callback(msg, {protocol: 'UDP', address: rinfo.address, port: rinfo.port, local: {address: address, port: port}});
+      var targetPort = 5060;
+      if(msg.headers.via[0].params.hasOwnProperty('sent-by'))
+        targetPort = msg.headers.via[0].params.hasOwnProperty('sent-by');
+      if(msg.headers.via[0].params.hasOwnProperty('rport'))
+        targetPort = msg.headers.via[0].params.rport;
+
+      callback(msg, {protocol: 'UDP', address: rinfo.address, port: targetPort, local: {address: address, port: port}});
     }
   }