The Titanic sent two different distress calls.
On the night of 15 April 1912, the Titanic's wireless operators first sent the older Marconi signal CQD, then the new SOS. The Carpathia, about 93 kilometres away, heard the call and rescued roughly 710 people. In July 1912, US law required passenger ships to carry two operators and keep a continuous radio watch. Mandatory maritime radio began that night.
Useful: Emergency rules are almost always written after the disaster.
Source: en.wikipedia.org