It doesn't have to prove you were at the scene. It only has to persuade the JURY beyond a reasonable doubt. It doesn't even have to eliminate all doubt. Just eliminate the reasonable ones.
Basically, if you claim you were home alone, but your phone was at the crime scene, you're in a world of hurt. Why was your phone at the crime scene? Did you lend it to someone? Did you lose your phone? When/where did you lose your phone? If you have no alibi, you're sunk.