The whole point is that the Postal Police were coming because the neighbor had found the cell phones belonging to MK in her garden, and thought they might be bomb devices.
The Postal Police were thus wholly unexpected. In the original theory supporting guilt, Knox and Sollecito had NO plans to phone police- they would wait for Filomena to do it.
Then, due to the Postal Police's awkward and untimely arrival, they had to phone police to impress the Postals as being honest and on the ball.
Obviously, if Sollecito really was calling first, unprompted by PP, this shows real consciousness of innocence. To me, as I said , this is the light and the key to all.
It all hinges on details such as this, and other bits such as if Amanda bought cleaning supplies first thing ......Then all snaps into place for guilt. Otherwise, it becomes very, very dubious. And then the other side begins to gain real ground.