Summary:

The SIP response 480 Temporarily Unavailable indicates that a device (IP phone, ATA, softphone, etc.) has received the call but is refusing it temporarily. Unlike registration errors, this happens after the device is registered and reachable, but before the call can be presented to the user.

When the 480 Comes From the Device

If a SIP trace shows the 480 Temporarily Unavailable originating from the endpoint device (not the PBX or upstream carrier), it means the device is explicitly rejecting the call for one of the following reasons.

Common Device-Level Causes

1. Do Not Disturb (DND) Enabled

   • Many devices respond to inbound calls with a 480 code when DND is active instead of the standard 486 Busy Here.

   • Look for a DND icon or menu option on the device screen.

   • Example Reason Header:

     vbnet
     Copy code
     Reason: Q.850; cause=47; text="DND Activated"
     

2. All Line Appearances Busy / No Call Waiting

   • If all configured lines are in use and call waiting is disabled, the device will reject additional calls with a 480 response.

3. Call Forwarding Failure

   • Forwarding to an unreachable or invalid destination can trigger an immediate 480 from the device.

4. Softphone / Mobile App in Sleep Mode

   • Mobile SIP apps may be suspended by the OS when not active in the foreground, causing them to send 480 for new calls.

5. Time-Based Call Restrictions

   • Some devices block calls outside configured “work hours” or during scheduled quiet times, responding with a 480.

6. Firmware or Resource Issues

   • High CPU load, memory limits, or firmware bugs may cause intermittent 480 responses.

How to Troubleshoot

1. Check Device Display & Settings

   • Verify DND is off.

   • Check for active call forwarding or time-based restrictions.

2. Review Call Trace Details

   • Look for Reason headers or other custom text in the 480 response.

   • Example:

     vbnet
     Copy code
     SIP/2.0 480 Temporarily Unavailable
     Reason: Q.850; cause=47; text="DND Activated"
     

3. Test with Known Good Conditions

   • Disable forwarding and DND.

   • Ensure at least one line appearance is free.

   • Retest the call.

4. Check Device Logs/Web UI

   • Most IP phones allow access to a local log showing call rejection reasons.

5. Update Firmware

   • If the behavior is sporadic or unexplained, install the latest firmware for the device model.

Quick Diagnostic Flow

• Is DND enabled? → Disable it and retest.

• Are all lines busy? → End one call and retest.

• Forwarding set? → Remove forwarding and retest.

• Using a softphone? → Keep it active in the foreground or enable push notifications.