SIP 100 Trying vs 480 Temporarily Unavailable
SIP 100 (Trying) is a 1xx Provisional response, while 480 (Temporarily Unavailable) is a 4xx Client Failure response. 100 indicates that the request has been received by the next-hop server and an unspecified action is being taken to process it. The UAC should expect additional responses. In contrast, 480 means that the callee's end system was contacted successfully but the callee is currently unavailable. The client may retry after some time.
Description
The request has been received by the next-hop server and an unspecified action is being taken to process it. The UAC should expect additional responses.
When You See It
Immediately after sending an INVITE. The proxy has received the call and is searching for the callee. This is the SIP equivalent of a dial tone.
How to Fix
No fix needed — this is normal call setup behavior. If stuck on 100 Trying for too long, check network connectivity and proxy configuration.
Description
The callee's end system was contacted successfully but the callee is currently unavailable. The client may retry after some time.
When You See It
When the user exists but is not currently reachable — phone is off, DND mode enabled, or all lines are busy.
How to Fix
Try calling again later. Check if the callee has DND enabled or if their device is powered off. Configure voicemail for unavailable scenarios.
Key Differences
100 is a 1xx Provisional response, while 480 is a 4xx Client Failure response.
SIP 100: The request has been received by the next-hop server and an unspecified action is being taken to process it. The UAC should expect additional responses.
SIP 480: The callee's end system was contacted successfully but the callee is currently unavailable. The client may retry after some time.
You encounter 100 when immediately after sending an INVITE. The proxy has received the call and is searching for the callee. This is the SIP equivalent of a dial tone.
You encounter 480 when when the user exists but is not currently reachable — phone is off, DND mode enabled, or all lines are busy.
When to Use Which
For 100 (Trying): No fix needed — this is normal call setup behavior. If stuck on 100 Trying for too long, check network connectivity and proxy configuration. For 480 (Temporarily Unavailable): Try calling again later. Check if the callee has DND enabled or if their device is powered off. Configure voicemail for unavailable scenarios.