If you’re seeing an error message or your SMS messages aren’t being delivered as expected, this article covers the most common causes and solutions.
⚠️ Note:
The list below includes the most common 18 error codes (16 from Twilio and 2 internal errors).
These are the ones most likely to appear when sending or receiving SMS via SleekFlow.
For all other codes, see this full list of error and warning.
Common SMS error codes
Error Code |
Description |
Possible Cause & Suggested Solution |
N/A |
Reached usage limit |
You have exceeded your internal plan’s message quota or rate limit. Wait until reset or upgrade. |
N/A |
Contact opted out |
This contact has unsubscribed from SMS messaging. Do not send further SMS. |
20003 |
Permission denied / authentication failed |
The request couldn’t be authorized — your Twilio credentials (Account SID, Auth Token, API Key) may be invalid, expired, or revoked. Check and re-enter them, ensure the correct environment (prod vs dev), and confirm your account is in good standing. |
21211 |
Invalid “To” number |
The destination number is malformed or not in E.164 format. Make sure you include country code (e.g. +1, +886) and verify it’s a mobile number. |
21408 |
Region not enabled for SMS |
You may not have enabled the destination country in your Twilio Geo-Permissions settings. Go to the Twilio console, enable SMS for that region, and try again. |
21606 |
“From” number not valid for SMS |
The Twilio number you’re using may not be SMS-capable or might not be allowed to send to the destination region. Use a number that supports SMS in that country. |
21608 |
Destination number not verified (for trial accounts) |
In trial mode, Twilio only allows messaging to numbers you’ve verified. Verify the destination number in Twilio, or upgrade out of trial mode. |
21610 |
Recipient has opted out (STOP) |
This contact has explicitly unsubscribed from SMS (they sent “STOP”). You must stop sending messages unless they opt back in. |
21611 |
Too many messages queued |
You have exceeded the message queuing capacity or are sending too fast. Reduce throughput, batch or space out sends, or use a messaging service with higher capacity. |
21614 |
“To” number cannot receive SMS |
The target number is a landline or otherwise incapable of receiving SMS. Use a mobile number. |
21617 |
Message too long |
Your message exceeds Twilio’s maximum of 1600 characters (including concatenation). Shorten your message or split it into parts / send via MMS if needed. |
30001 |
Message queue overflow / sending too fast |
The SMS queue is overwhelmed. Try sending another message later, use batching, or increase your throughput limits. |
30003 |
Handset unreachable |
The recipient’s device may be turned off, out of coverage, or temporarily disconnected. Retry later. |
30004 |
Message blocked by carrier |
The message was filtered (spam rules, policy violation). Simplify content (avoid URLs or promotional language), verify compliance, and retry. |
30005 |
Unknown destination handset |
Carrier indicates the number is invalid or no longer active. Check and remove inactive numbers. |
30006 |
Landline or not supported |
The number is a landline or otherwise unable to receive SMS. Switch to a mobile number or use another channel. |
30007 |
Carrier filtering / policy violation |
The content or frequency triggers filtering. Adjust your messaging strategy, reduce frequency, remove suspect content. |
30008 |
Unknown delivery error |
The carrier returned a generic failure. Retry later; monitor logs; contact Twilio if persistent. |