Carrier Blocking / Rate Limiting

Carrier Blocking and Spam Detection

The phone carriers (ATT, Sprint, Verizon, etc) all have their own distinct rules and algorithms for determining if outbound messages are “spammy” in nature. Those algorithms take into account message content, outbound rate of delivery, number of customers that opt-out following the receipt of a message, and the rate of bi-directional engagement on an SMS number.

If carriers detect “spammy content” they may deny delivery of messages. In extraordinary circumstances, they may even blacklist a phone number and deny delivery of all outbound messages on a specified SMS channel to their customer base.

The carriers are all independent entities. Being rate-limited or marked as spammy on the Verizon network doesn’t necessarily mean that Sprint will mark the same messages as spammy.

Please note that some carriers will incorrectly and purposefully report to Prompt.io that messages that are carrier-blocked were actually delivered. Prompt.io may have very little visibility into the viability of outbound messages that are carrier blocked.

Messages delivered via long-code (10 digit phone numbers) are much more susceptible to spam filtering than messages delivered via short code (5 or 6 digit numbers) and toll free numbers.

Keys to Minimizing Carrier Blocking

Rate Limiting

In order to maximize successful delivery, the Prompt.io platform will limit outbound text messages sent via bulk messaging to one message every four seconds (for messages delivered via Twilio). This is best practice for minimizing carrier blocking.

At one message every 4 seconds, Prompt.io will process approximately 15 in a minute, or around 1000 customers an hour. As an example, an outbound message to 5000 customers will take 5 hours to complete from end to end.

If you have a very high number of customers or a compressed timeframe for delivery, we suggest segmenting the audience into multiple lists and performing outbound texting over multiple SMS outbound numbers while varying the message body for each segment.

5000 customers spread into 10 lists of 500 each and delivered via 10 outbound numbers would complete delivery in about 30 minutes.

Please note that if we have land-line enabled your phone number through our partnership with Zipwhip, we facilitate message delivery at 1 message per second. The threshold for spam on these land-line enabled channels is lower and a higher throughput is possible.

Additional Resources

Please contact client services if you have any questions about best practices, rate limiting, land-line texting, or carrier blocking.