Overview

SIP ALG (Application Layer Gateway) is a feature built into most routers and firewalls. Its original purpose was to help SIP (Session Initiation Protocol) traffic pass through NAT (Network Address Translation) by inspecting and modifying SIP packets. In practice, however, SIP ALG causes far more problems than it solves, especially with modern Hosted PBX systems that already handle NAT traversal on their own.

One of the most common casualties of SIP ALG interference is BLF (Busy Lamp Field), the feature that lets phones display the real-time call status of other extensions. When SIP ALG tampers with the signalling messages that power BLF, the result is unreliable presence information, phantom busy indicators, and general confusion for your team.

What Is BLF and How Does It Work?

BLF (Busy Lamp Field) is a monitoring feature on IP phones that displays the live call status of other extensions. It typically appears as a set of indicator lights or line keys on a desk phone, or as a status panel in UC Operator or UC Advanced Softphone.

How BLF communicates behind the scenes

BLF relies on the SIP SUBSCRIBE/NOTIFY mechanism. Here is how the conversation works between your phone and the Hosted PBX:

1. SUBSCRIBE: Your phone sends a SIP SUBSCRIBE message to the Hosted PBX, essentially saying, "I want to watch Extension 201 and let me know when their status changes."
2. 200 OK: The Hosted PBX acknowledges the subscription.
3. NOTIFY: Whenever Extension 201 picks up or ends a call, the Hosted PBX sends a SIP NOTIFY message back to your phone with the updated status (idle, ringing, on a call, etc.).
4. Refresh: The subscription periodically renews, so your phone continues to receive updates.

This SUBSCRIBE/NOTIFY cycle is the heartbeat of BLF. If any part of it gets corrupted or lost, BLF stops working correctly.

What Is SIP ALG and Why Is It a Problem?

SIP ALG is a "helper" function in routers that attempts to inspect SIP packets as they pass through NAT and rewrite IP addresses and port numbers inside the SIP headers. The idea was to solve connectivity issues, but in reality, SIP ALG often does the following:

• Rewrites Contact headers: Changes the IP address or port your phone advertises, potentially misdirecting return traffic.
• Modifies Via headers: Alters the routing path information, making it impossible for the Hosted PBX to reply to the correct location.
• Corrupts SDP content: Breaks the media description section of SIP packets, potentially affecting audio streams.
• Mangles SUBSCRIBE/NOTIFY messages: Rewrites the very messages BLF depends on, causing status updates to get lost or delivered to the wrong destination.
• Drops or duplicates packets: Some SIP ALG implementations silently discard packets they cannot parse, or create duplicate registrations.

How SIP ALG Specifically Breaks BLF

When SIP ALG is active on a network running your Hosted PBX phones, the following BLF issues commonly appear:

Broader Impact on SIP Signalling

BLF is just one victim. SIP ALG can disrupt the full range of SIP signalling on your Hosted PBX system. Here are other common issues caused by the same root problem:

Registration failures

Phones randomly unregister or display "No Service" because the SIP ALG rewrites REGISTER messages, causing the Hosted PBX to lose track of the phoneis location on the network.

One-way or no audio

SIP ALG modifies the SDP (Session Description Protocol) portion of INVITE messages, changing the media IP addresses. The call connects at the signalling level, but audio is sent to the wrong destination, resulting in one-way sound or complete silence.

Call transfer and hold failures

Features like call transfer, call hold, and three-way calling rely on additional SIP methods (re-INVITE, REFER). SIP ALG often mishandles these, causing transfers to drop or held calls to disconnect.

Voicemail and auto-attendant issues

If SIP ALG corrupts the signalling for call forwarding to voicemail, callers may hear dead air or be disconnected before reaching the greeting.

Impact on UC Operator and UC Advanced Softphone

Software-based clients, such as UC Operator (for reception consoles) and UC Advanced Softphone, are especially sensitive because they rely on accurate, timely SUBSCRIBE/NOTIFY traffic to display real-time call status panels. SIP ALG interference makes these tools unreliable.

How to Diagnose SIP ALG Issues

If you suspect SIP ALG is causing BLF or signalling problems, follow these steps:

1. Check the router or firewall settings. Log into the network device and look for a setting labelled "SIP ALG," "SIP Transformations," "SIP Helper," or "SIP Fixup." Different manufacturers use different names for the same feature.
2. Compare SIP traces. Capture a SIP trace from the phone side and compare it to a trace on the Hosted PBX side. If the Contact, Via, or SDP fields differ between the two, SIP ALG is rewriting packets in transit.
3. Look for mismatched ports. If the Hosted PBX Portal shows a different contact port than what the phone is configured to use, that is a strong indicator of SIP ALG activity.
4. Test with SIP ALG disabled. The fastest diagnostic is simply to disable SIP ALG and reboot the router. If BLF and other issues resolve, you have your answer.

The Fix: Disable SIP ALG

The recommended resolution is straightforward: disable SIP ALG on any router or firewall sitting between your phones and the internet. Here is how to do it on common devices:

After Disabling SIP ALG: Verification Steps

1. Reboot the router to clear any cached SIP ALG state.
2. Reboot all IP phones so they send fresh REGISTER and SUBSCRIBE messages.
3. Check phone registration in the Hosted PBX Portal. All phones should show as registered with the correct contact IP and port.
4. Test BLF by having a colleague make and receive a call. Watch the BLF indicators on the monitoring phone to confirm they update in real time.
5. Test call quality by making inbound and outbound calls. Verify two-way audio, call transfer, and call hold.
6. Monitor over 24 to 48 hours to ensure BLF subscriptions are refreshing correctly and no intermittent issues return.

Quick Reference Summary