They had gotten "Tip line" calls from pay phones in the area, and the nubmer they were given (in the message) to be called on was also in that area. But I agree, no way was the sniper, or the leaver of the note and/or the tipline caller would use a phone in that area. Much policework depends on the bad guy being dumb, as most of them are, but this guy or these guys are apparently not.
The longer this drags on the more it seems we aren't even in the loop. I find it hard to believe that one or two people can drive around and kill at will, any time or place with all the heat that's on, especially when you challenge them.
This has to be something big, and we may never know what.
I'm going with terrorism on this one( kicking back in armchair).