IANAL, but my understanding is that obstruction of justice via spoliation, tampering, or destruction of evidence is a charge that requires your investigation to have already begun, the raid to have already started, or the arrest to have been made, and that you are free to destroy any of your own property prior to these events.

Specifically, you need to knowingly be the subject of an investigation. I'd assume destroying the phone when you see the cops coming but before you've seen the warrant would be a grey area. Please, lawyers, clarify.