SMS Carrier Filtering in Canada and US
SMS Carrier Filtering in Canada and US
Section titled “SMS Carrier Filtering in Canada and US”SMS filtering is applied by two parties: wireless carriers and SIPSTACK. This article explains how each works and what steps you can take to maintain deliverability.
Background: A2P Long Code Messaging
Section titled “Background: A2P Long Code Messaging”Historically, Application-to-Person (A2P) business messaging over 10-digit long code numbers was not supported by carriers in Canada and the US — it was reserved for short codes. This changed when US carriers introduced A2P 10DLC registration, allowing registered businesses to send A2P traffic over long code numbers with reduced filtering risk.
Canadian carriers have not introduced A2P 10DLC solutions. Canadian long code messaging remains subject to carrier filtering based on content and volume patterns.
How Carrier Filtering Works
Section titled “How Carrier Filtering Works”Carriers in Canada and the US use adaptive (machine learning) systems to filter messages. These systems evaluate:
- Volume: How many messages have originated from a phone number during a given time period
- Content: Whether message contents resemble known spam patterns
- Opt-out signals: High STOP reply rates or user-reported spam complaints
This behaves similarly to email spam filtering — cumulative signals trigger a block rather than any single factor.
Filtering by Wireless Carriers
Section titled “Filtering by Wireless Carriers”Carriers filter for two main reasons:
- Subscriber protection — Blocking spam and unwanted messages before they reach end users
- A2P enforcement — Detecting unregistered A2P traffic being sent over long code numbers
When carrier filtering occurs, affected messages are returned with a status of Undelivered and error code 30007.
Filtering by SIPSTACK
Section titled “Filtering by SIPSTACK”SIPSTACK’s filtering targets traffic that violates the Acceptable Use Policy. SIPSTACK filtering is designed to allow compliant A2P messaging through. As long as recipients have provided consent and are not generating spam complaints or high STOP rates, SIPSTACK filtering should not affect your traffic.
Avoiding Carrier Filtering
Section titled “Avoiding Carrier Filtering”| Practice | Why It Matters |
|---|---|
| Adhere to SIPSTACK’s Acceptable Use Policy | The foundation of compliant messaging |
| Only message opted-in recipients | Reduces complaint rates and STOP replies |
| Include opt-out instructions in the first message | ”Reply STOP to unsubscribe” is required by carriers |
| Register for A2P 10DLC (US long code) | Registered traffic has lower filtering risk on AT&T, Verizon, and T-Mobile |
| Verify your toll-free number (if applicable) | SIPSTACK offers toll-free verification to reduce filtering on compliant traffic |
Getting a Blocked Number Unblocked
Section titled “Getting a Blocked Number Unblocked”If you believe your messages are compliant but are being filtered (error 30007):
- Collect 3 or more Message IDs (SSIDs) with
Undeliveredstatus and error 30007. - Contact SIPSTACK Support with those examples.
- SIPSTACK will investigate and, where the carrier accepts appeals, submit a request for review.
Carrier Appeals
Section titled “Carrier Appeals”Major US carriers handle appeals differently:
| Carrier | Appeal Process |
|---|---|
| AT&T | Accepts appeals for false-positive filtering on registered A2P traffic |
| Verizon | Accepts appeals for false-positive filtering |
| T-Mobile | Appeals process available for registered A2P 10DLC traffic |
Pre-Approval: Currently Unavailable for Long Code
Section titled “Pre-Approval: Currently Unavailable for Long Code”Carriers in Canada and the US do not pre-approve messages from long code numbers. Short code numbers are allowed for specific traffic types (and are unlikely to be filtered when you adhere to your approved use case). Toll-free numbers can be verified through SIPSTACK to proactively reduce filtering risk.