One proposed method for getting ETI's attention is the supernova hail. We make the assumption that ETI is interested in astronomy, and like to observe supernovae as much as we do. We also might assume they are aware of the supernova hail method too.

When we see a supernova, we calculate the point directly opposite its coordinates in the sky. And as soon as we can, we fire a coherent, tight beam of maser radio or laser light in that direction. Let's say it takes us a week or so to get our equipment ready to do so.

Years, maybe centuries later, ETs from all over the galaxy start picking up the supernova, and slew their 'scopes around to it to study it., depending on when the wavefront gets to them. Those who are directly opposite the SN from us, will see, about a week later, the faint signal of our beacon superimposed on it. Other civilizations scattered about the galaxy will not see our tight beam because they are not lined up with it, but the guys who do will know there is a signalling world between them and the supernova.

From our point of view, every time we detect a supernova, we should continue monitoring that spot on the celestial sphere for coherent radio or light beacons for some time after the SN dies down, in the hope ET is signalling us in this fashion. The supernova is used as a means of getting someone's attention.

Since several extragalactic SNs occur every year, we could be sending several hails a year into our own galaxy. Of course, the big assumption here is that ET is interested in supernova studies.