Most Common Cause for US NumbersIncomplete A2P 10DLC registration. This is required for all US business SMS and causes silent delivery failure when not completed.
Diagnose in Order
1
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.
2
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.3
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.
4
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.
5
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.
6
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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