SIP ALG Causing Call Drops on Grandstream Phones: How to Fix It

SIP ALG (Application Layer Gateway) is a router feature designed to help with Hosted Voice traffic, but it often causes the opposite effect. On Grandstream endpoints connected to your Hosted PBX, SIP ALG is one of the most common causes of intermittent call drops, one-way audio, and failed transfers. This guide walks you through confirming the issue and resolving it quickly.

Step 1: Confirm SIP ALG Is the Likely Cause

Look for these patterns in the reported issue:

• Calls drop after exactly 30 or 60 seconds
• One party cannot hear the other after the call connects
• Transfers or holds fail inconsistently
• The problem affects multiple phones on the same network

If two or more of these match, SIP ALG is a strong suspect.

Step 2: Disable SIP ALG on the Router

This step must be done on the customer's router, not the phones. The exact menu path varies by router brand, but the setting is usually found under:

Advanced Settings > NAT > SIP ALG (or similar)

Set SIP ALG to Disabled, then save and reboot the router. If the router does not expose this option, escalate to the customer's IT contact or internet provider, as some ISP-managed routers require a support call to disable it at the firmware level.

Step 3: Check Grandstream Phone Settings

After disabling SIP ALG on the router, confirm these settings on each affected Grandstream device via the phone's web interface (enter the phone's IP address into a browser):

Under Account Settings:

• NAT Traversal: Set to Keep-Alive
• STUN Server: Leave blank unless your Hosted PBX configuration requires one

Under Advanced Settings:

• Layer 3 QoS (DSCP SIP): 26
• Layer 3 QoS (DSCP Audio): 46

Save and reboot the phone.

Step 4: Test and Verify

Place several test calls, including transfers, holds, and calls lasting longer than one minute. Ask the customer to confirm behaviour over the next 24 hours, as intermittent issues may not surface immediately after a single test.

What to Expect Next

Most customers see full resolution after completing Steps 2 and 3. If call drops persist after the router reboot and phone changes, the next step is to capture packets to check for SIP signalling errors at the network level. Open a ticket with the support team and include the router make and model, Grandstream firmware version, and a call log showing the drop times. This gives the team everything they need to investigate without delay.