Phone System · 1 min read
Error 21635: "To" Number Is a Landline
What this error means
Error 21635 means the destination number is a landline and can't receive SMS through a standard mobile route. Unlike a formatting error, the number itself may be perfectly valid. The number type just doesn't support texting.
How to fix it
- Open the contact record and review the number.
- Confirm whether it's a landline.
- If the contact has a mobile number, add it to the record and use it for SMS.
- If there is no mobile number, reach the contact by phone call or email instead.
- Re-send the message only after the mobile number is saved.
Prevent it in automations
- Ask for a mobile number on forms where you plan to send texts.
- Add a branch in workflows so landline contacts get a call task or email instead of SMS.
- Keep office numbers and mobile numbers in separate fields when you collect both.