Error 21635 occurs when you attempt to send an SMS message to a phone number that is identified as a landline. Standard SMS delivery requires a mobile-capable destination number.
What Is Error 21635?
Error 21635 means the destination number in the To field is a landline and cannot receive SMS through a standard mobile messaging route.
This error is different from a general formatting issue because the phone number itself may be valid, but the number type does not support SMS.
Key Benefits of Identifying Landline Numbers
Checking number type before sending can help reduce avoidable messaging failures.
- Fewer Failed Messages: Prevent SMS attempts to destinations that cannot receive texts.
- Cleaner Contact Data: Separate mobile numbers from landlines more accurately.
- Improved Automation Reliability: Reduce workflow failures caused by unsupported destinations.
- Better Communication Routing: Use voice or another supported channel when SMS is unavailable.
Common Cause of Error 21635
The primary cause is:
- The destination phone number is a landline.
The number may still be valid and correctly formatted, but it is not eligible to receive SMS through a standard messaging route.
How to Fix Error 21635
Start by confirming whether the recipient has a mobile number available.
- Open the affected contact record.
- Review the destination number.
- Confirm whether the number is a landline.
- If the contact has a mobile number, update the contact record with the correct mobile number.
- Retry the SMS using the mobile number.
- If no mobile number is available, use another communication method such as a voice call or email.
Check Contact and Imported Data
If Error 21635 occurs during a workflow, campaign, or bulk send, review how the destination number was added to the contact record.
Common sources include:
- Imported contact lists
- Form submissions
- Manually entered contact information
- Custom phone fields
- Synced external data
- Older contact records where a business landline was stored as the primary phone number
Correcting the stored number can prevent repeat failures.
Consider Using Another Communication Channel
If the recipient only has a landline, SMS may not be available for that contact.
Depending on your communication needs, consider:
- Placing a voice call
- Sending an email
- Requesting a mobile number
- Updating the contact record when a mobile-capable number becomes available
Frequently Asked Questions
Q: What does Error 21635 mean?
It means the destination number is a landline and cannot receive SMS through the standard messaging route.
Q: Is the phone number invalid?
Not necessarily. The number may be valid, but its number type does not support standard SMS.
Q: Can I fix the error by changing the phone number format?
No. If the number is truly a landline, changing the formatting will not make it SMS-capable.
Q: What should I do if the contact has another number?
Use a valid mobile number for SMS and update the contact record accordingly.
Q: Can this happen in workflows or bulk campaigns?
Yes. Any automated message can return Error 21635 if the destination field contains a landline.
Q: Should I retry the same SMS to the same number?
No. Retrying the same message to a confirmed landline will not resolve the issue.
Need Help?
If Error 21635 continues, first confirm that the destination number is not a landline.
Verify that:
- The contact’s phone number is a mobile-capable number.
- The correct phone field is being used in the message or workflow.
- The number was not imported incorrectly.
- A business landline has not been stored as the contact’s primary SMS number.
- The contact record has been updated if a valid mobile number is available.
- The workflow or campaign is not repeatedly retrying the same landline number.
If the contact only has a landline, use another supported communication channel instead of SMS.
If the issue continues, collect the destination number, number type if known, contact record, workflow or campaign name if applicable, and a screenshot of Error 21635 before contacting support.