The first step to troubleshooting this issue is to attempt to replicate the problems. Attempt to send another test message to this user via a REST API request, or through the API Explorer in the Twilio Console.
If you see similar results, continue troubleshooting with the following checklist: Is the destination device powered on? Does the device have sufficient signal? If not power the device off, wait 30 seconds, and then power it back up. Is the device connected to the home carrier's network? We cannot guarantee message delivery on devices roaming off-network. Can the device receive non-Twilio SMS? Can the device receive messages from another Twilio number (non-Alphanumeric Sender ID), or with a shorter one-segment (non-concatenated) body? Can other devices using the same mobile carrier receive your messages?
If you can rule out all of the above issues, continue troubleshooting below.
Repeated Error 30003 results on undelivered messages sent from short codes could occur if the destination user has a wireless plan that does not support short code or "Premium" messages.
To workaround these plan limitations, we suggest using the Fallback to Long Code feature of Messaging CoPilot. By enabling this feature, CoPilot will attempt first to send your message out using your short code, and then make a second attempt over a long code if necessary. For more details, please see our blog post "Improve SMS Deliverability with Fallback to Long Code".
Sprint customers may be able to whitelist messages sent from your short code by texting the phrase
Allow XXXXX to 9999, replacing
XXXXX with your short code. However, customers from other wireless carriers will likely need to reach out to their carrier's support group to enable "Premium" or short code messaging.
Repeated Error 30003 results on undelivered messages sent from long codes (regular phone numbers) could be due to carrier filtering.
The rules for carrier filtering vary throughout the industry, making it difficult at times to pin down exactly why a message is being filtered out. For more details including potential workarounds, please see this Help Center article "How Does Carrier Filtering Work?"
Twilio's Support team can help investigate what went wrong with delivering your message. Please collect 3 or more message SIDs in your SMS logs from the last 24 hours that were flagged with Error 30003, and open a support request.