Typically, SIP INVITE messages fit within the standard Ethernet MTU. In cases where a large number of headers or metadata are included, an INVITE can exceed the MTU and be fragmented. While such large messages are ideally sent over TCP, some providers may still send them over UDP.
On 2026-01-08 06:11 UTC, we deployed updated firewall rules in our UK and EU regions as part of our regular security review. During this update, a configuration that allows fragmented packets was unintentionally omitted. As a result, larger SIP INVITE messages (inbound calls) sent over UDP were dropped.
This issue affected a limited subset of customers in the UK and EU regions, accounting for approximately 1.5% of total inbound calls in the affection regions during the highest point of traffic for the day.
The issue was identified and the firewall rules were rolled back at 2026-01-08 20:17 UTC.
We already maintain end-to-end tests that cover large, fragmented packets. However, these tests were previously run only in a subset of regions. To prevent similar incidents in the future, we are expanding these tests to run in every cluster and to execute immediately after firewall and security configuration updates are applied.