Signal is a communications platform for iOS and Android devices that supports text messaging, voice calls, and video calls. While users are identified by their phone number, Signal messages and calls are sent via the internet using a wifi or data connection, not over the cellular voice network. Because of this, there are no charges beyond data fees for Signal calls, even for long distance or international calls. Signal conversations are encrypted using the namesake Signal Protocol, designed by Open Whisper Systems and subsequently also adopted by WhatsApp, Facebook Messenger, and Google Allo. Unlike the other systems using the Signal Protocol, complete source code for the Signal client applications as well as the Signal servers is available under a free software license. Signal has been recommended by the Electronic Frontier Foundation (EFF) in their "Surveillance self-defense guide," has been endorsed by Edward Snowden, and was recently approved by the US Senate's Sergeant at Arms for official use by Senate staff members.