Why SMS Messages Are Not Delivering in MONDAi

Diagnose and fix SMS delivery failures in MONDAi — from A2P registration to invalid numbers and carrier filtering.

Warning

Most Common Cause for US Numbers

Incomplete A2P 10DLC registration. This is required for all US business SMS and causes silent delivery failure when not completed.

Diagnose in Order

Check A2P 10DLC registration status (US only)
Go to Settings → Phone Numbers → A2P Registration. Status should show Approved. Pending or Rejected status is the most common cause of SMS failures for US numbers.
Verify the phone number format
Phone numbers must be stored in E.164 format: +1XXXXXXXXXX for US numbers. A number stored as (555) 123-4567 may fail silently. Check the contact record for the correct format.
Check if the contact has opted out
Search the contact record for an opt-out or unsubscribed status. Contacts who replied STOP will not receive SMS messages — this is by design and legally required.
Test with a different phone number
Send a test SMS to your own number. If it delivers to you but not the customer, the issue is specific to their number or carrier.
Check for carrier filtering
Some carriers filter messages from numbers that have not registered their business. This is separate from A2P and affects certain campaign types.
Check the conversations inbox for errors
After a failed send, go to the contact's conversation thread. Error messages from Twilio (the underlying SMS provider) appear inline and contain specific failure codes.

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