This document discusses the over-the-top (OTT) messaging landscape. It reviews various messaging apps like iMessage, Skype, WhatsApp, and Facebook and how they are embedded in other services. It notes that while early instant messaging in the 1990s-2000s did not kill SMS, the commercial launch and interconnectivity of SMS between carriers in 1995-1998 is what really boosted its adoption. The document also outlines some business models for current OTT apps like advertising, payments, partnerships with tech companies, and integration with other services. It emphasizes that national cross-platform interoperability is crucial for users and that silos between OTT apps pose a challenge.