SIP ALG can cause issues if enabled on Routers, Modems or in rare cases a managed switch.
Most manufacturers enable SIP ALG as default on devices, 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.