On October 27, 2021, Twilio experienced an incident involving high filtering of SMS messages directed towards the AT&T network in the United States. The issue began with engineers investigating high filtering on SMS traffic.

The problem manifested as increased filtering on SMS messages, impacting Twilio’s Programmable Messaging service. Specifically, SMS Long Code, SMS Short Code, and SMS Toll-Free services in North America were affected, with toll-free numbers experiencing persistent filtering even after some recovery for other message types.

The root cause was identified as an issue with a carrier partner, specifically AT&T’s network, which was applying high filtering to SMS traffic. Twilio engineers collaborated with their carrier partner to address and resolve the problem.

Customers attempting to send SMS messages to recipients on the AT&T network in the United States likely experienced delays or failures in message delivery due to the increased filtering.

Twilio’s remediation efforts involved continuous monitoring and working directly with the carrier partner. Recovery was observed throughout the day, and the incident was fully resolved when high filtering ceased for all senders towards the AT&T network.