The 911 system records the phone # of land lines AND cell phones. They already know who you are when you call. Questions are asked to make sure it isn’t pranked. In this case, the cell phones are compared to the people at the scene. Zimmerman was told to back off, he chose not to thus he is the one who caused the shooting.
Do they *voiceprint*, too? Just a number and a few questions doesn’t seem to be quite enough as “documentary evidence”, IMO.