Resolving SMS Delivery Failures Due to Invalid or Inactive Numbers

What’s Happening

When you send an SMS to a number that’s invalid, deactivated, or a landline, carriers block the message and your dashboard logs a failure (21211: Invalid 'To' Phone Number | Twilio, 21614: 'To' number is not a valid mobile number - Twilio).

How to Fix It

1. Locate Failed Messages

   • In your SMS platform, open the Delivery Failures view. Look for messages marked with error codes 21211 or 21614 (Error and Warning Dictionary | Twilio).

2. Check the Error Code

   • 21211 means the number was formatted incorrectly or is not recognized.

   • 21614 means the number is not a mobile line (often a landline or IoT device) (21211: Invalid 'To' Phone Number | Twilio, 21614: 'To' number is not a valid mobile number - Twilio).

3. Validate the Number

   • Confirm each number uses E.164 formatting (+ + country code + subscriber number) and isn’t on any Do-Not-Originate (DNO) list (Phone number validation best practices - Twilio).

4. Notify Your Contact

   “It looks like the number [phone] is invalid or inactive, which is why your message didn’t deliver. Please verify or update this number so we can resend your SMS.” (Failed Message Processing Fees - Twilio Help Center)

5. Correct and Retry

   • Update or remove bad entries, then resend your campaign. For large lists, use a bulk validation API to check numbers in advance (Twilio (SMS) - SuprSend, Notification infrastructure for Product teams).

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Best Practices for Maintaining a Clean SMS Contact List

Why You Need It

A clean list ensures your messages reach real, engaged recipients—saving you money and protecting your sender reputation (SMS List Hygiene & Best Practices, SMS List Cleansing & Data Validation Essentials from our Experts).

Your Action Plan

• Real-Time Validation:
  Use input masks and validation scripts at signup to catch typos and non-mobile numbers immediately (7 Best Practices for Validating Phone Numbers in E-Commerce).

• Scheduled List Cleansing:
  Quarterly or monthly, remove numbers with repeated delivery failures or long periods of inactivity (SMS List Cleaning: Why Is Is Important and How to Do It | TxtCart).

• Double Opt-In:
  Send a confirmation SMS (“Reply YES to confirm”) before adding contacts to your marketing lists to ensure number ownership (SMS List Cleansing & Data Validation Essentials from our Experts).

• Segmentation & Personalization:
  Group contacts by behavior or demographics so you send only relevant messages, reducing opt-outs and complaints (Best practices for phone and email verification - Infobip).

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📊 Understanding Common SMS Error Codes

Error Code	What It Means	How to Handle
21211	Invalid ‘To’ Phone Number	Check for typos and ensure E.164 format; remove any short-code or alphanumeric IDs ([21211: Invalid 'To' Phone Number
21614	Not a Valid Mobile Number	Verify line type via Lookup API; exclude landlines/IoT numbers from SMS lists (21614: 'To' number is not a valid mobile number - Twilio).
21610	Message Blocked (Unsubscribed)	Customer has replied STOP; guide them through resubscription or use a preference center (Twilio (SMS) - SuprSend, Notification infrastructure for Product teams).
30003	Undelivered: Destination Unreachable	Handset off or network issue; suggest retry later or alternative channel (Twilio (SMS) - SuprSend, Notification infrastructure for Product teams).
30005	Undelivered: Number Unknown/Deactivated	Number may have been ported or deactivated; remove and request updated contact info (Failed Message Processing Fees - Twilio Help Center).