Error 21211 occurs when an outbound call or message is sent to a phone number that is invalid, incomplete, or incorrectly formatted. The destination number must use E.164 format, which includes a plus sign, country code, and full subscriber number.

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What Is Error 21211?

Error 21211 means the destination phone number entered in the To field could not be recognized as a valid phone number.

The expected format is:

+[country code][area code][subscriber number]

For example:

+15551234567

Using the correct format helps ensure calls and messages are routed to the intended destination.

Key Benefits of Correct Phone Number Formatting

Using properly formatted destination numbers helps prevent failed calls and messages.

  • Fewer Delivery Failures: Correct formatting reduces avoidable call and messaging errors.

  • Improved International Routing: Country codes help route communications to the correct destination.

  • More Reliable Automations: Workflows and campaigns are less likely to fail because of invalid contact data.

  • Cleaner Contact Records: Standardized phone numbers are easier to manage and troubleshoot.

Common Causes of Error 21211

Error 21211 can appear when the destination number cannot be processed correctly.

Common causes include:

  • The phone number is not in E.164 format.

  • The country calling code is missing or incorrect.

  • The destination number contains a typo.

  • The same phone number is being used as both the sender and recipient.

  • A message is being sent to a short code.

  • A message is being sent to an alphanumeric sender ID.

  • The destination number contains an unsupported or invalid prefix.

  • A workflow, form, import, or contact record contains malformed phone data.

How to Format a Phone Number Correctly

Use E.164 formatting whenever possible.

A valid number should contain:

  • A + symbol

  • The country calling code

  • The area or regional code, when applicable

  • The complete subscriber number

  • No unnecessary spaces, brackets, or local dialing prefixes


For example:

Correct: +14155552671

Incorrect: (415) 555-2671

Incorrect: 4155552671

Incorrect: 0014155552671

How to Fix Error 21211

Review the affected destination number before trying the call or message again.

  1. Open the contact or destination number that generated the error.

  2. Confirm the phone number belongs to the intended recipient.

  3. Add the correct international country code.

  4. Format the number using E.164 format.

  5. Remove unnecessary local dialing prefixes.

  6. Confirm the sender and recipient numbers are not identical.

  7. Make sure the destination is a standard phone number rather than a short code or alphanumeric sender ID.

  8. Save the corrected number.

  9. Retry the call or message.

Check the Country Calling Code

Incorrect country codes are a common cause of Error 21211.

When sending internationally, confirm that the destination number includes the correct country calling code.

For example:


  • United States and Canada: +1

  • United Kingdom: +44

  • Australia: +61

Do not assume the contact's current location based only on the area code. Use the country associated with the actual phone number.

Check for Sender and Recipient Number Conflicts

A phone number generally cannot place a call or send a message to itself.

If the From and To numbers are identical:

  • Select a different outbound number, or

  • Correct the recipient number if it was entered incorrectly

This can happen when contact records, workflows, or test messages use the wrong phone field.

Check for Short Codes and Alphanumeric Sender IDs

Short codes and alphanumeric sender IDs are not standard destination phone numbers.

If you are trying to send a message to one of these destinations, the message may fail with Error 21211.

Confirm that the To field contains a valid, full phone number in E.164 format.

Check Automated Workflows and Imported Contacts

If Error 21211 occurs during an automated workflow, bulk send, or campaign, the problem may be stored in the contact record rather than the message itself.

Review:

  • Imported phone numbers

  • Form submissions

  • Custom fields used as destination numbers

  • Workflow phone-number fields

  • Contact records with incomplete country codes

  • Numbers copied from spreadsheets or external systems

Standardizing phone numbers before using them in automation can reduce repeated failures.

Frequently Asked Questions

Q: What does Error 21211 mean?
It means the destination phone number is invalid or incorrectly formatted.

Q: What format should I use for phone numbers?
Use E.164 format: a plus sign, country code, and complete phone number.

Q: Can I send a message to the same number I am sending from?
No. The sender and destination numbers should not be the same.

Q: Can a short code be used as the destination number?
Not through a standard phone-number sending flow. Use a valid destination phone number.

Q: Why does the number look correct but still fail?
The number may have the wrong country code, an unsupported prefix, hidden characters, or incomplete digits.

Q: Can imported phone numbers cause this error?
Yes. Imported numbers that are missing country codes or contain invalid formatting can trigger Error 21211.

Need Help?

If Error 21211 continues after correcting the destination number, first review the phone number format and the source of the contact data.

Verify that:

  • The destination number is in E.164 format.

  • The correct country calling code is included.

  • The number contains all required digits.

  • There are no typos, extra characters, or unsupported prefixes.

  • The sender and recipient numbers are different.

  • The destination is not a short code.

  • The destination is not an alphanumeric sender ID.

  • The correct phone field is being used in the workflow, campaign, or contact record.

  • Imported or form-submitted phone numbers were saved correctly.

  • The number is valid for the destination country.

If the issue continues, collect the affected destination number, sender number, country, communication type, workflow or campaign name if applicable, and a screenshot of the Error 21211 message before contacting support.