Can My SMS Messages or Phone Numbers Be Blocked as Spam?

← General

Can My SMS Messages or Phone Numbers Be Blocked as Spam?

Yes. Wireless carriers always reserve the right to filter incoming SMS and MMS messages from certain numbers. They do this to protect their subscribers from spam, unwanted messaging, and fraud.

Why Messages Get Blocked

Carriers use adaptive (machine learning) filtering systems that evaluate both message content and sending volume. A number or message thread can be flagged and blocked based on:

• High send volume from a single number in a short time period
• Message content that resembles known spam patterns
• High opt-out rates — too many recipients replying STOP
• End user complaints reported through carrier spam reporting tools
• Unregistered A2P traffic — business messages sent over unregistered long code numbers in the US

Caution

Carrier filtering can happen without warning. If you see a spike in Undelivered statuses with error 30007, your number or content may have been flagged.

How to Reduce the Risk of Being Blocked

1. Only send to opted-in recipients. Obtain clear, documented consent before messaging anyone.
2. Include opt-out instructions. Always include “Reply STOP to unsubscribe” in your first message to any recipient.
3. Avoid content that resembles spam. Do not use excessive capitalization, suspicious links, or messages that pattern-match known spam templates.
4. Do not snowshoe. Spreading the same message content across multiple numbers to avoid detection is a known tactic that carriers specifically target.
5. Register your traffic (US only). A2P 10DLC registration with The Campaign Registry reduces filtering risk for business messaging over 10-digit long code numbers. See What is A2P 10DLC?
6. Toll-free numbers must be verified. Carriers block texts from unverified toll-free numbers. Self-serve verification is not available in Switchboard yet — contact SIPSTACK Support to ask about it.

Getting Unblocked

If you believe your messages are compliant with SIPSTACK’s Acceptable Use Policy and are still being filtered:

1. Collect 3 or more Message IDs (SSIDs) with Undelivered status and error 30007.
2. Contact SIPSTACK Support with those examples.
3. SIPSTACK will investigate and, where supported by the carrier, submit an appeal.

Note

Most carrier block lists use a “cooling off” period — numbers are automatically removed from the block list after some time. The exact duration varies by carrier and is not shared with SIPSTACK. If message content does not also change, content-based filtering will continue.

Related Resources

• SMS Carrier Filtering in Canada and US
• What is A2P 10DLC?
• Forbidden Message Categories
• Support for Opt-Out Keywords