April 19, 2023

A Comprehensive Guide to Call Blocking Notifications: Understanding SIP Codes 603, 607, and 608

Delve into SIP codes 603, 607, and 608 to provide you with a thorough understanding of their implications on-call delivery and your Caller ID reputation.

A Comprehensive Guide to Call Blocking Notifications: Understanding SIP Codes 603, 607, and 608

Table of Contents

  1. Introduction
  2. Overview of SIP Protocol
  3. SIP Response Codes Explained
  4. SIP Code 603: Decline
  5. SIP Code 607: Unwanted
  6. SIP Code 608: Rejected
  7. Impact on Caller ID Reputation
  8. How to Improve Call Delivery and Reduce Blocking
  9. Conclusion


As the telecommunication industry evolves, businesses must adapt to maintain effective communication with their customers. One critical aspect of this adaptation involves understanding call-blocking notifications and the various SIP codes associated with them. In this comprehensive guide, we will delve into SIP codes 603, 607, and 608 to provide you with a thorough understanding of their implications on-call delivery and your Caller ID reputation.

Overview of SIP Protocol

The Session Initiation Protocol (SIP) is an essential component of Voice over Internet Protocol (VoIP) technology, enabling the establishment, modification, and termination of multimedia sessions. SIP is responsible for initiating and managing communication sessions between two or more parties, such as voice and video calls.

SIP Response Codes Explained

SIP response codes are numerical codes that indicate the outcome of a SIP request, such as an attempted call. These codes are grouped into six classes, based on the first digit of the code:

  • 1xx (Provisional): Request received, continuing to process
  • 2xx (Successful): The action was successfully received, understood, and accepted
  • 3xx (Redirection): Further action needs to be taken to complete the request
  • 4xx (Client Error): The request contains bad syntax or cannot be fulfilled
  • 5xx (Server Error): The server failed to fulfill an apparently valid request
  • 6xx (Global Failure): The request cannot be fulfilled at any server

SIP Code 603: Decline

The SIP code 603, also known as "Decline," indicates that the recipient has rejected the call. Reasons for this rejection may include the recipient's call-blocking settings, a busy line, or simply a decision not to accept the call. In most cases, the decline is temporary and does not necessarily imply a permanent block. However, repeated instances may negatively impact your Caller ID reputation.

SIP Code 607: Unwanted

SIP code 607, or "Unwanted," signifies that the call recipient has marked the call as unwanted, often due to spam or telemarketing concerns. This code may be triggered by various factors, including the call's content, the frequency of calls, or the recipient's personal preferences. A high volume of calls resulting in the 607 code can severely damage your Caller ID reputation and lead to increased call blocking.

SIP Code 608: Rejected

The "Rejected" SIP code, 608, indicates that the call has been blocked by the recipient's service provider due to a poor reputation or suspected malicious activity. This code often results from a history of unwanted or spam calls originating from the same Caller ID. Persistent instances of SIP code 608 can lead to long-term call blocking and significantly impact your business's ability to communicate with customers.

Impact on Caller ID Reputation

Caller ID reputation plays a crucial role in determining whether your calls are delivered successfully or blocked by carriers and recipients. A poor Caller ID reputation, often resulting from a high number of declined, unwanted, or rejected calls, can lead to increased call blocking and reduced call delivery rates. It is vital for businesses to monitor and maintain a healthy Caller ID reputation to ensure effective communication with their customers.

How to Improve Call Delivery and Reduce Blocking

To improve call delivery rates and minimize call blocking, consider implementing the following best practices:

  1. Monitor and analyze call data: Regularly review your call data to identify trends and potential issues related to call blocking and SIP codes. This analysis can help you pinpoint specific areas for improvement and take corrective action.
  2. Verify and maintain accurate Caller ID information: Ensure that your Caller ID information is accurate, up-to-date, and properly formatted. Inaccurate or misleading information can lead to increased call blocking and a negative Caller ID reputation.
  3. Comply with regulations and industry standards: Adhere to all relevant telemarketing regulations, such as the Telephone Consumer Protection Act (TCPA) and the Do Not Call (DNC) registry, to avoid legal penalties and maintain a positive reputation.
  4. Avoid spam-like behavior: Refrain from using aggressive or intrusive calling practices that may be perceived as spam or unwanted calls. This includes excessive call frequency, calling during inappropriate hours, and using pre-recorded messages without consent.
  5. Implement call authentication protocols: Adopt call authentication standards like STIR/SHAKEN to help verify your calls' legitimacy, reducing the likelihood of call blocking and improving call delivery.


Understanding and addressing call-blocking notifications, specifically SIP codes 603, 607, and 608, is crucial for businesses that rely on telecommunication to connect with their customers. By following best practices and maintaining a healthy Caller ID reputation, you can improve call delivery rates, reduce call blocking, and ensure seamless communication with your clients.

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