Who says that ETI will use radio signals?
Let's not forget Optical SETI, which was briefly popular in the early 1960s and revived in the late 1990s:
http://www.coseti.org/content1.htm
As for other methods ETI might use to contact us, see here:
http://www.coseti.org/lemarch1.htm
http://www.setileague.org/editor/lde.htm
