Fallback is the automatic switch from a preferred, more secure transport to a less secure one when security requirements cannot be met. In mobile messaging, that can mean moving from encrypted RCS to plain SMS if the app, carrier, device, or eligibility checks do not support end-to-end encryption.
This matters because users may think a conversation is protected when it has silently dropped to a weaker path. Attackers benefit from fallback when they can force or exploit downgrade conditions, while defenders use it as a reliability mechanism so messages still deliver instead of failing. Security teams should treat fallback as a trust boundary issue: verify which protocol is actually in use, watch for downgrade warnings, and avoid assuming that a modern app automatically means encrypted traffic. A safe design makes fallback explicit, visible, and as limited as possible.