since you want to test the cell phone not the application, which means it relates to testing of hardware components of a cell phone.
As it deals with hardware testing there is no long such methodologies which we used for working with application. A small or simple plan is enough to proceed with.
Testing cell phone (not application) has the following things has to be check with
1) checking battery
2) checking display ( display quality and in case of touch screen check the sensitivity)
3) checking functionality of buttons (includes buttons like power button etc)
4) checking enhancements ( charges, headset, data cable)
5) checking quality of the components ( which includes panel etc)
these are the basic things but i might have missed many...