This report indicates that there was a system in place that could have warned Sri Lanka, India, and countries in Africa of the impending tsunami.
But. It’s not manned on public holidays.
And the system is only available for some countries.
History first. This system that could be used to warn of impending disasters was originally started in 1997 to monitor clandestine nuclear testing. It’s based in the US in the Berkshires.
"But most of the network’s monitoring stations are unmanned, particularly during holidays. And the agency that runs the network is bogged down in Cold War nuclear sensitivities about who sees its data. But besides its technical attractions, the network also relies on instant communications, using a database of private mobile telephone numbers of key points of contact in all the countries hit by the tsunami. So there would be no excuse that nobody knows who to warn, as we’ve heard in soul-searching over the Sumatran disaster."