I have a black moto razr...
Black Razr. Droooll...
Without knowing the composition of the phones shell, its hard to day. Start gentile, the work up from there. Me, I'd start with a bit of grain alcohol on a soft cloth, then on to dilute Windex if the ETOH did not work, again on a soft cloth.
Else, just live with it. Tech gets used, and there comes a point where personal tech aquires character with the evidence of its use.