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Troubleshooting Delivery Issues

Srikanth Chinnammagari
Organization

Overview

This Knowledge Base article provides definitions, common causes, and recommended actions for frequently encountered SMS delivery error codes. These errors are typically returned by mobile carriers or messaging platforms and indicate why a message could not be delivered successfully. 
This guide is intended to assist support teams in diagnosing delivery failures for Long Code (10DLC) and Toll-Free messaging traffic.

 

Error Codes – Long Codes (10DLC)

Error Code 503

Description: 
Occurs when a customer sends a message from a long code that is not associated with any campaign or is not linked to an active 10DLC campaign ID and when the status is showing Rejected with providerStatusID 503.

Common Causes:

  • No 10DLC campaign associated with the sending number

Recommended Actions:

  • Verify the sending number is properly linked to an active 10DLC campaign
  • Ensure the campaign status is active and deployed

 

Error Code 15

Description: 
Message was sent to:

  • A prepaid mobile number with insufficient funds
  • An invalid postpaid number
  • A subscriber who has opted out of SMS

Recommended Actions:

  • Verify the destination number and subscriber status
  • Ask the recipient to confirm account balance and SMS eligibility with their carrier

 

Error Code 045

Description: 
May indicate a velocity block.

Common Causes:

  • Carrier rate-limiting or spam prevention controls
  • Non-compliant message type or sending behavior

Recommended Actions:

  • Reduce the message sending rate
  • Review traffic patterns and campaign throttling settings
  • Open a ticket with the vendor to remove the block
  • Note: Always consult the compliance team before opening a ticket with the Syniverse support team

 

Error Code 66

Description: 
Message undelivered due to one or more of the following.

Common Causes:

  • Recipient is not an AT&T subscriber
  • Prepaid user has insufficient balance
  • Subscriber is blocked from receiving this type of SMS

Recommended Actions:

  • Verify recipient carrier and account status
  • Confirm message type compliance

 

Error Code 00B

Description: 
SMS destination number is disabled or belongs to a subscriber who has opted out of receiving SMS messages.

Recommended Actions:

  • Confirm the destination number is active
  • Verify opt-in status before retrying message delivery

 

Error Codes – Toll-Free Numbers

Error Code 4b6

Description: 
Subscriber-level rejection requiring carrier intervention.

Common Causes:

  • Account status issues
  • A2P (Application-to-Person) blocking
  • Provisioning problems at the carrier level

Recommended Actions:

  • Advise the subscriber to contact their mobile carrier to resolve the issue

 

Error Code 4ce

Description: 
ESME Receiver Reject Message error. This error is 100% associated with AT&T.

Common Causes:

  • Message rejected at AT&T’s network level
  • Carrier-specific filtering or policy enforcement

Recommended Actions:

  • No platform-side remediation available
  • Review message content and sending patterns
  • If the issue persists, escalate with AT&T carrier support

 

Error Code 45c

Description: 
Recipient is not SMS-enabled. Traffic returns status 1007: FAILED, indicating handset or subscriber-level issues.

Common Causes:

  • Handset not provisioned for SMS
  • Carrier billing or account restrictions

Recommended Actions:

  • Confirm with the recipient that their number supports SMS
  • Ask the recipient to check with their carrier for billing or provisioning issues

 

Error Code 456

Description: 
User has opted out of receiving messages from this number.

Common Causes:

  • End-user previously sent an opt-out keyword (for example, STOP)

Recommended Actions:

  • Messages will continue to fail until the user explicitly opts back in
  • If re-engagement is required, ensure the recipient opts back in following carrier consent rules

 

Error Code 3ef

Description: 
Same category as 4b6. Carrier returned 1206: Invalid Destination Address at the subscriber level.

Common Causes:

  • Carrier provisioning issue
  • A2P block
  • Invalid subscriber status

Recommended Actions:

  • Subscriber must contact their carrier to resolve the issue

 

Error Code 488

Description: 
Toll-Free number is not verified with the Toll-Free Hub (ZipWhip).

Common Causes:

  • Toll-Free verification not completed or not approved

Recommended Actions:

  • Complete Toll-Free number verification via the Toll-Free Hub
  • Ensure approval is received before sending traffic

 

Summary Table – Quick Reference

Error Code Primary Cause Action Required
503 No active 10DLC campaign Link number to active campaign
15 Insufficient funds / invalid number Verify account and balance
045 Velocity block Reduce send rate
66 Carrier or balance restriction Verify carrier and balance
00B Disabled or opted-out number Confirm number status
4b6 Subscriber/carrier issue Subscriber contacts carrier
4ce AT&T network rejection Carrier-driven, review content
45c Not SMS-enabled Verify handset and billing
456 User opted out User must opt back in
3ef Invalid destination address Subscriber contacts carrier
488 Toll-Free not verified Complete TF verification

 

If you would like, I can also:

  • Convert this into a customer-facing KB article
  • Adapt it for internal Tier-1 / Tier-2 support use
  • Create a troubleshooting decision tree or flowchart for faster diagnosis

 

 

 

 

 

  • Error code 503 – A 503 error occurs when a customer sends a message from a number that is not associated with any campaign. This error also appears if the number is not linked to an active 10DLC campaign ID.
  • Error code 4b6 - Subscriber needs to contact their carrier to resolve (account status, A2P block, or provisioning issue).
  • Error code 4ce - ESME Receiver reject message error – 100% AT&T. 
    This rejection comes from AT&T and indicates the message was not accepted at their network level. Such rejections can vary in reason but are generally carrier-driven.
  • Error code 45c - Meaning: Recipient is not SMS-enabled — the traffic returned status 1007: FAILED indicating handset/subscriber issues. 
    Possible causes: handset not provisioned for SMS, or carrier billing/account issue on the destination side. 
    Action: Confirm with the recipient (or their carrier) whether the number supports SMS and that there are no account/billing restrictions.
  • Error code 456 - Meaning: User opted out. End-user previously opted out from receiving messages from this number. 
    Action: Messages will continue to fail until the user explicitly opts back in. If re-engagement is required, the recipient must opt back according to carrier/consent rules.
  • Error Code 15 – Message was destined to a prepaid mobile with insufficient funds, an invalid postpaid number, or to a subscriber who opted out of SMS.
  • Error Code 045 – Can sometimes indicate a velocity block.
  • Error Code 66 – Undelivered due to: 
    • Not an AT&T subscriber 
    • Prepaid user with insufficient balance 
    • Subscriber blocked from receiving this type of SMS
  • Error Code 00B – SMS destination number is either disabled or belongs to a subscriber who has opted out of receiving SMS messages.
  • Error code 3ef - Meaning: Same category 4b6 — carrier returned 1206: Invalid Destination Address (subscriber-level). 
    Action: Subscriber needs to contact their carrier to resolve (account status, A2P block, or provisioning issue).
  • Error code 488 – When the Toll-Free Number is not verified with the Toll–Free Hub (ZipWhip).

 

 

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