US SMS Carrier Filtering Mechanisms and How to Resolve Each (2026)

US SMS carriers filter outbound messages for various reasons. The main filtering mechanism is requiring registeration for senders (A2P 10DLC, toll-free, short-code) before the first message can even go out. But there are other filtering mechanisms in place too. This article lists these filters and how to resolve them.

Before you start

Product teams that use Pingram typically don’t face these issues, because unlike Twilio or Telnyx, Pingram correctly submits your registerations (A2P and others) and sets up your telecom infrastructure automatically vs. requiring you to learn and handle it alone. Learn more about Pingram SMS.

The mechanisms

This is a comparison of filters, not a delivery-rate ranking. The same block uses a different number on Telnyx and on Twilio.

Mechanism	What it blocks	Error codes	How to resolve it
A2P 10DLC	Long-code traffic with no approved campaign on that number	40010, 30034	Complete A2P registration by submitting your brand, then campaign. Pingram handles this for you.
Toll-free	US or Canada traffic from a toll-free number that is not verified	30032, 30442	Complete toll-free verification Pingram handles this for you.
Spam	Content or a pattern the carrier treats as unwanted	40002, 30007	Review what you are sending. Ensure your API keys or system is not being abused by spammers.
Bitly and public shorteners	A link that hides the destination	30963	Put the link on a domain you own.
Not routable	A landline, a 555 number, or a number the carrier will not accept	40001, 40012, 10002, 30006, 21211, 30005, 21614	Not possible to text this number. Try a different number or channel.
Unreachable	A mobile that is off or out of coverage	30003	Retry with backoff.
Undeliverable	The carrier refused a wireless number and did not say why	40008, 30008	Retry. Repeated failures on numbers that otherwise deliver are abuse filtering.
Expired	The message aged out before the handset got it	40005, 30036	Resend only if the content is still valid.
STOP	The recipient opted out	40300, 21610	Wait for START.
Volume	Traffic past the campaign class	(4)780	File the class for the volume you send or switch to Pingram which automatically handles this.

A2P 10DLC

40010 and 30034 mean the long code is not attached to an approved 10DLC campaign, so carriers drop the traffic. Register the brand and campaign, attach the number you send from, and match the samples, opt-in, and privacy policy to the live text. If you want to do this yourself, refer to our Ultimate A2P 10DLC Guide. If you prefer to skip these requirements and focus on shipping faster, Pingram handles this for you.

Toll-free

30032 means the toll-free number is not verified, so US and Canada traffic is blocked. 30442 means verification was rejected because the use case looked like spam. Finish verification before production traffic, or send that traffic from a registered 10DLC number. Pingram supports both on one account.

Spam

40002 and 30007 mean the carrier or the provider filtered the content or the sending pattern. Match the live text to the registered samples, name the business, and keep a real opt-out. If compliant messages still fail, open a support case with three or more undelivered message IDs.

Bitly and other public shorteners

This is usually caused by your content containing links from public shorteners (bit.ly, tinyurl.com, or a domain that only redirects) which are heavily abused for phishing. Replace your links with URLs on a domain you own.

Volume

Each registration (A2P, toll-free, short-code) has a volume cap based on the brand, type of registration, and recipient’s telecom operator. These errors mean you are over the cap. If you are not using Pingram, find the limit, file the appropriate campaign, register a new brand, or open a ticket to review your volume.

If you are using Pingram, you won’t have to worry, because the Pingram team and software automatically handles the necessary compliance for your caps to increase as your traffic grows.

If you want to learn more about these caps, refer to our 10DLC Throughput Limit on a Low Volume Campaign (2026) blog.

Not routable

40001, 40012, 10002, 30006, 21211, 30005, and 21614 mean the carrier will not route to that number: a landline, a 555 number, or a wireless number it has marked invalid. We recommend removing the number or reaching the customer through an alternate channel (Pingram supports emails and voice too).

Unreachable or Undeliverable or Expired

30003 means a mobile handset did not respond, because it was off or out of coverage. 40008 and 30008 mean the carrier refused and did not name a rule. 40005 and 30036 means the message expired while the carrier was delivering it.

Don’t remove the number, instead continue sending. You can retry the same message with a backoff (recommended to be hours).

You may also receive these errors if you are sending too many messages to the same number, so your messages are being delayed or filtered to avoid abuse. You may want to check if you’ve been sending too many messages to the same person. This keeps happening if you are a developer testing an SMS feature with your own number.

STOP

40300 (HTTP 409) and 21610 mean the recipient texted STOP, so further SMS to that number is blocked. The next send that will go through is after they text START. On Pingram, STOP suppresses later SMS to that number, including the same notification, and START turns them back on.

It’s recommended that you stop attempting to send to the user, so you are not incurring more SMS fees. Depending on your application’s terms of use, you may be allowed to continue sending through another channel like Email if you have their email opt-in, and if their SMS STOP does not imply an email opt-out.