In a Fermax MEET system, the SIP ALG (Application Layer Gateway) parameter on some routers and firewalls can cause abnormal behaviour because it attempts to automatically modify SIP packets to facilitate VoIP telephony. However, MEET already correctly handles SIP signalling and multimedia traffic, so these modifications often result in abnormal behaviour.

Typical cases where SIP ALG causes interference

1. The MEET system is working correctly with one operator, but upon switching to another operator, monitors with extensions or external numbers begin to receive calls from unknown sources.

1. The handset receives a large number of calls.
   
2. The handset’s call log shows that it has received a large number of calls.
   
3. The call log shows nonsensical names and numbers.
   

Cause: SIP ALG was enabled, and not all operators handle SIP in the same way; some filter SIP ports 5060–5061 or apply specific VoIP policies

2. In a home with more than one monitor, the incoming call rings on the main monitor (0), whilst the other slave monitors do not receive the call

Symptoms:

1. The main monitor receives the call.
   
2. Secondary monitors do not receive the call nor can they power on automatically.
   
3. We disconnected the router, left the MEET devices connected to an unmanaged switch, and all terminals worked correctly.
   

Cause: SIP ALG is interfering with the MEET system’s SIP signalling.

3. Incoming call with no audio or one-way audio

Symptoms:

1. The monitor receives the call.
   
2. The connection is established.
   
3. Only one party can hear the audio.
   

Cause: SIP ALG modifies the IP addresses and SDP ports contained in the SIP INVITE and OK messages, causing the RTP audio streams to be sent to the wrong address.

4. No video or black screen

Symptoms:

1. The call connects correctly.
   
2. There is audio but no video.
   
3. The image appears for a few seconds and then disappears.
   

Cause: By altering the SDP negotiation, the SIP ALG modifies the ports used for RTP or RTCP video, preventing the video stream from reaching the monitor or the MEET server correctly.

5. Door opener not responding

Symptoms:

1. The call is working.
   
2. The door-opening button appears on the screen.
   
3. The door-opening command does not reach the door control board.
   

Cause: Some SIP ALGs filter or alter SIP INFO, MESSAGE or re-INVITE messages used during the session for certain auxiliary functions.

6. Intermittent SIP registration

Symptoms:

1. Monitors appearing and disappearing as online.
   
2. SIP registrations expiring prematurely.
   
3. Random disconnections.
   

Cause: The SIP ALG modifies REGISTER messages or server responses, causing inconsistencies between the router’s NAT tables and MEET’s SIP server.

7. Calls that drop after 30–60 seconds

Symptoms:

1. The call connects correctly.
   
2. After a fixed period of time, it disconnects.
   

Cause: The SIP ALG does not correctly maintain NAT associations or alters SIP session refresh messages (Session Timer).

8. Problems with call forwarding or calls to mobile devices

Symptoms:

1. The board calls the local monitor.
   
2. Call forwarding to a mobile device fails or is erratic.
   
3. Inconsistent push notifications.
   

Cause: Changes made by SIP ALG may affect communication between the SIP server and external services or devices located outside the local network.

9. Problems with VPN connections between buildings

Symptoms:

1. Several MEET installations connected via VPN.
   
2. SIP registrations work but calls do not.
   

Cause: SIP ALG attempts to modify private IP addresses within the VPN tunnel, corrupting the signalling information that is already valid for the remote network.

How to identify a potential SIP ALG issue

1. Disconnect the router and ensure that communications between MEET devices take place via the unmanaged switch.
   
1. Check that communications are working correctly.
   
2. If the same symptoms persist whilst connected solely to the switch,
   
1. check that the switch provided by Fermax (ref. 1586) has its tab set to ‘Standard’; if it is set to ‘Extended’, communication may fail.
   

Recommendation for MEET installations Fermax recommends disabling SIP ALG on routers and firewalls, particularly on routers from:

1. Mikrotik
   
2. Ubiquiti
   
3. TP-Link
   
4. DrayTek
   
5. Zyxel
   
6. Asus
   
7. Cisco
   

Conclusion:

SIP ALG is one of the most common causes of abnormal behaviour in MEET IP systems when operator routers or network equipment that modifies SIP signalling are involved.

Therefore, the first recommended step when dealing with call or SIP registration issues is to check that this function is disabled on the router or firewall.