Knowledgebase

SIP 1xx codes are provisional responses.

They’re not errors, but early indications during a SIP transaction, typically sent after an INVITE but before a 200 OK. These responses give the caller status updates while the call is still being processed.

They play an important role in proper SIP communication, and mishandling or incomplete support of these responses can lead to one-way audio, dropped calls, or inconsistent call setup.

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Common SIP 1xx Responses Explained

100 Trying
The receiving server is performing a search or lookup before routing the call.
A 100 Trying is normally sent by the first server to receive the INVITE, especially if it’s a proxy, to prevent retransmission by the caller.
➡️ This does not reach the end user, but helps manage timeouts and transaction state.

180 Ringing
The destination User Agent (UA) has received the INVITE and is alerting the user.
You’ll often hear ringing start at this point. This response is forwarded through SIP proxies and is important for the caller’s experience.

181 Call is Being Forwarded
The call is being redirected, often to another SIP URI or number.
The SIP proxy or server sends this to inform the caller that redirection is happening. Useful for call forwarding features.

182 Queued
The destination is temporarily unavailable, maybe busy or overloaded.
The call has been queued server-side, and the system may send multiple 182 responses to update the caller while waiting.
This is useful in call centre queues or time-based routing scenarios.

183 Session in Progress
Used to indicate that media is being set up, often before the call is answered.
This allows early media (e.g. announcements, ringback tones, or IVR prompts) to be played.
➡️ The called party may not be alerted yet, but media streams are already active.
Proper 183 handling requires compatible codec negotiation and support for early media in both endpoints and the provider’s infrastructure.

199 Early Dialog Terminated (optional, not widely implemented)
Used by a User Agent Server (UAS) to inform upstream SIP entities, including the User Agent Client (UAC), that an early dialog (created by a 1xx response) has ended.
➡️ Useful when multiple forks were created, and one of them gets cancelled before reaching 200 OK.

⚠️ Why These Might Cause Trouble

These codes are part of the call setup flow, but:

• Some platforms or firewalls may block or mishandle 183, causing no audio

• Devices without early media support may miss ringback tones or announcements

• Misconfigured proxies may not forward all 1xx responses properly

• Improper handling of 100 Trying or 199 may lead to confusion in multi-path routing

That’s why your VoIP provider matters.

✅ Why Use Voyced for SIP Calls?

At Voyced, we’ve designed our platform to handle all SIP signalling correctly, including INVITE, early dialogs, and forking behaviour.
Our Hosted IPPBX and SIP trunking support:

• Full and proper early media (183 Session in Progress)

• Real-time call status updates (100/180/182)

• Global DID support in 165+ countries

• A–Z Termination with Premium Plus routing

• Built-in call queues, IVRs, voicemail and more, no need to build this manually

• Clear CLI (Caller Line Identification) with proper SIP headers

So you get fewer issues, better visibility, and calls that connect as expected.

Still seeing problems with early call states?

Mismanaged 1xx responses can make your platform feel unreliable, but it doesn’t have to be that way.

Want fewer problems and more peace of mind?

Switch to Voyced now

and enjoy better SIP signalling, from INVITE to OK, every time.

Last updated: 01/04/2025

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Given the fast evolution of the telecom sector, Voyced does not offer any guarantee about the accuracy and completeness of the information offered. 
We explicitly do not give anyone permission to use this information in any other way than to inform themselves about what (legal) requirements there may be and they need to comply with when registering Services and/or DIDs with Voyced.