A SIP ALG can and usually does exist on Routers, Modems or in rare cases a managed switch.
You must disable this feature for any VoIP service to operate as expected.
Client Based Testing
This link will download a small .exe agent that can test for the presence of a SIP ALG on the network the host PC is connected to..
After downloading, open the file and accept any permissions your windows/mac machine may ask for.
If the results = False then a SIP ALG was not detected by the client.
If the results = True then a SIP ALG is active and must be turned off.
Mac OS X Based Testing
This link will download a small APP agent that can test for the presence of a SIP ALG on the network the host MAC is connected to..
After downloading, open the DMG, move the contents to Applications and accept any permissions your mac machine may ask for.
Web Based Testing
Visualware is a provider of internet based VoIP testng tools.
They offer a series of VoIP test tools that include a SIP ALG tester.
Below are 2 links to some of their web based test.
These web test will simulate 1 VoIP Call using the 711 codec and provide a realtime analysis of that call.
Note: That analysis ONLY represents the quality of that single call and should not be used as a benchmark for real calling scenarios with multiple active calls during an extended period of time.
After the test completes click on the ‘VoIP’ tab and look for ‘N‘ or ‘Y‘ on the ‘SIP ALG Firewall‘ line in the lower white box.
If you see “SIP ALG Firewall: Y” then a SIP ALG is active and must be turned off.
If you see “SIP ALG Firewall: N” then a SIP ALG is was not detected.