SIP ALG (Session Initiation Protocol Application Layer Gateway)

SIP ALG (Session Initiation Protocol Application Layer Gateway) helps overcome issues related to Network Address Translation (NAT) by allowing SIP sessions to pass through routers. It dynamically modifies information in SIP packets to ensure proper routing through NAT. However, incorrect configuration or SIP ALG incompatibility can cause problems, and in such cases, disabling it is recommended.

SIP ALG Detector

An SIP ALG is typically present in Routers, Modems, or occasionally in a managed switch.
To ensure proper operation of any VoIP service, it is necessary to deactivate this function.

Links:
(Windows) - Used to check networks for SIP ALG
(Mac OS) - Used to check networks for SIP ALG

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.

More links:
SIP-ALG-Detector
SIP-ALG-Detector c#

Testing via the Web

Visualware provides internet-based VoIP testing tools.
They offer a range of VoIP testing utilities, including a SIP ALG tester.
Below are two links to some of their web-based tests.

This option must be run using internet explorer with java enabled.
This option runs using HTML5 and their custom plugin.

Instructions:
These web tests simulate a single VoIP call using the 711 codec and deliver real-time analysis of that call.

Note:
The analysis provided represents only the quality of that individual call and should not be regarded as a benchmark for real calling scenarios involving multiple active calls over an extended period.

After the test is completed, navigate to the 'VoIP' tab and check the 'SIP ALG Firewall' line in the lower white box for either 'N' or 'Y'.
If it shows "SIP ALG Firewall: Y", then SIP ALG is active and needs to be disabled. If it shows "SIP ALG Firewall: N", then SIP ALG was not detected.