Understanding What It Means When a Text Message to a Lead Is 'Undelivered'

Text deliverability issues can impact your ability to connect with leads—especially if carriers mark messages as spam. This article clarifies:

Sierra integrates with Twilio to send individual and bulk text messages through the Dialer.
Twilio is a top provider of SMS services.
Deliverability issues are not caused by Sierra or our account setup. Even if multiple clients use Sierra numbers under the same Twilio account, this does not impact filtering.
Once a message leaves Sierra and Twilio, delivery is controlled by the mobile carrier.

A message marked “Undelivered” appears in the lead detail page with a Twilio error code (e.g., 30003, 30004, etc.).
The system note includes the error code and a link to this article for context.

30003	Unreachable destination handset
30004	Message blocked
30005	Unknown destination handset
30006	Landline or unreachable carrier
30007	Carrier violation
30008	Unknown error

Cell carriers actively filter commercial SMS, especially high-volume or repetitive messages.
Carriers do not provide detailed rules for filtering, and filtering is outside Sierra and Twilio control.
Filtering is often per-message, but once a number is flagged, future messages—even unique ones—may be more likely to be filtered.
Deliverability may fluctuate: the same lead can receive some messages successfully and others marked undelivered.

Repetitive or templated content can trigger classification as automated application-to-person (A2P) traffic.
Once a number is flagged by carriers, its reputation is impacted, increasing the likelihood of future undelivered messages.
Using short codes is not recommended: they are expensive and clients prefer local numbers to communicate with leads.

The system displays the error code for undelivered messages on the lead detail page.
Twilio Lookup identifies the line type (mobile, landline, VOIP) when a lead is first routed through the system.
- Lookup is applied automatically for all new messages if missing.
- Manual imports or older leads may not have Lookup initially.
These measures provide visibility into undelivered messages and help avoid sending SMS to unsupported numbers.

Avoid repetitive or templated content in bulk SMS.
Space out messages, especially from new numbers.
Prioritize active leads and reduce unnecessary bulk sends.
Limit aggressive high-volume texting in short windows.
Rotate or replace Dialer numbers showing high undelivered rates (small forwarding fee applies).

High percentage of undelivered messages
Try retiring and forwarding from the affected Dialer number. Use a new number and reduce outbound SMS volume.
Lead never receives texts despite appearing valid
Check the Line Type using Twilio Lookup in the lead detail. If it’s a landline or VOIP, SMS may not be supported.

Is Sierra’s system causing messages to fail?
No. Messages leave Sierra and Twilio successfully—delivery failures occur at the carrier level, based on filtering.
Are we charged for filtered messages?
Yes. Twilio bills Sierra for every message sent, regardless of carrier filtering.
Will all undelivered messages have an error code?
Yes. Sierra displays Twilio’s error codes in the system note.
Can I mark a lead as opted out after an undelivered message?
No. Since many leads receive some messages successfully, auto-opting out would risk blocking real communication.

Learn why some texts are marked “Undelivered,” what causes it, and how to reduce filtered messages.