My theory on that is that the 911 call is the Sanford Police calling themselves in order to discover the phone's number. If the phone was passworded, the police would not have been able to examine its directory entries and logs without first breaking the password. However, passworded phones can be used to call 911 without entering the password. If I'm correct, it's odd that it took them until 2 March to get around to it.
If the police had the phone as evidence from the scene, then how did Tracy get the phone later? Don’t they have to keep evidence?