Seeing a SIP 603 “Decline” Error? Here’s What To Do

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It’s a fairly common scenario—you attempt to make a phone or video call just as you’ve done a hundred times before. This time, rather than connecting to your destination, a “SIP 603 ‘Decline’ Error” message pops up on your screen. With nothing but a cold, hard digital readout standing between you and your intended call recipient, it’s easy to feel frustrated and more than a bit confused.

Not to worry. Your system is simply telling you that your call could not be completed. There are a handful of reasons this may occur, the most common being that the recipient has rejected your call—but that isn’t always the case.

In order to discern the specific problem behind this type of system error, you’ll need to dig a little deeper. Here, we’ll help you decode this message and get to the bottom of the issue.

SIP 603: What’s Really Going On

Session Initiation Protocol (SIP) is a basic building block of modern Voice over Internet Protocol (VoIP) phone systems. As its name suggests, this series of codified parameters both initiates and terminates every VoIP phone or video communication.

In the context of traditional landline networks, it’s helpful to imagine SIP as the switchboard operator powering the successful connection between you and your destination. Through a process that takes mere seconds, SIP ensures that your voice or video data gets where it needs to go.

When you click or tap your screen to place a call, SIP signals to the network that you’d like to transmit multimedia data between your unique user port—or connection point—and that of the person you’re trying to reach.

Differences between SIP and VoIP graphic from Nextiva.

To initiate this process, SIP sends a request to the recipient’s port asking to make a connection. This request includes important details such as your identity, the identity of the person or people you’re calling, and the type of data you’re sending.

In turn, the destination port signals a response that determines whether or not your transmission can proceed.

During this time, you may receive a variety of SIP response codes indicating your call progress. Some of the most common include a “100: Trying” message, a “180: Ringing” message, or a “200: Ok” message signaling that your call connected successfully.

SIP response codes can range from the 100s to the 600s, with each tier referencing a different category of possible connectivity outcomes.

All codes in the 600 range are categorized as “global error” messages indicating that your call cannot go through. A “603: Decline” typically conveys that you successfully reached the recipient’s port but they declined to accept the transmission.

Remember those landline calls where, instead of a familiar ring, you’d hear a series of beeps followed by a woman stating, “Your call cannot be completed as dialed”?

A 603 code is today’s version—only it doesn’t always mean that the recipient declined your call. Although it’s generally known as a “603: Decline” error, this message actually just means that your call could not be completed.

One possible reason might be that your recipient has blocked your number or declined to answer. However, the issue may also originate within your own phone system, in which case you can troubleshoot it fairly quickly by running through a series of simple steps.

Common Fixes for SIP 603 Response

If your call is being declined by the recipient, there isn’t much you can do on your side, apart from trying back later or attempting to reach out through an alternate contact method such as email.

Since a 603 code doesn’t specifically detail where the problem lies, you can also check for issues on your side. It’s possible that you don’t have adequate system permission to access a person’s port. This can usually be remedied by requesting an update of your network role by your organization’s IT administrator.

Another reason for a 603 message might be that your phone or device time-out settings are different from those of the Private Branch Exchange (PBX) network you’re trying to reach.

In that case, you can try updating your phone settings accordingly and attempt to call again. It’s also possible that your destination PBX isn’t enabled to accept the specific data type you’re seeking to transmit. You might try an alternative method—a voice call instead of video, for example—and see if that goes through successfully.

On occasion, you may receive a 603 code because your number has been incorrectly flagged as spam and is therefore rejected by the recipient’s network. This can occur for a variety of reasons. Your number may have previously been associated with a consistently high call volume or reported by someone as spam, even before it was assigned to you.

Another explanation might be that your system caller ID settings need to be updated so your port requests can be properly identified.

These issues can be identified and addressed via your system interface or by contacting your VoIP provider for assistance.

In certain cases, your VoIP provider itself might be the source of the error. If your account balance is low or your subscription isn’t sufficient to manage the call, it will return a declined message. To remedy this, you can try troubleshooting specific VoIP-related issues as a good starting point. You can also verify your account funds online through your service interface or by contacting your provider.

SIP 603 vs. Other Global Failure Responses

All SIP “6xx” response codes indicate specific types of errors or failures in the data transmission connection process. The 603 code represents that your call cannot be completed and is usually fixed by running through a series of troubleshooting steps. Other common 6xx-level codes can include:

  • 600: Busy Everywhere indicating the recipient’s port was reached successfully but that the person is busy.
  • 604: Does Not Exist Anywhere signaling that certain information contained in the SIP request is incomplete or invalid.
  • 606: Not Acceptable indicating that the recipient is open to receiving transmission but cannot support the requested session—possibly due to an incompatible or outdated system.

You should note that while certain SIP error codes might seem a bit vague, they are part of an ever-evolving online system. As such, they’re constantly being refined and updated to better suit the needs of end users.

One such update, implemented in late 2022, expands the 603 error code to “603+” and includes additional codified details about where the call was blocked and why. Whether or not you receive a detailed error message, you can follow the same troubleshooting steps to rectify the problem and reach your intended destination.

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