Could i power the mobile phone without battery , only using 5Vdc supply ?
if it's possible how i connect to the mobile power pins, i know that there are 3 pins 2 of them 0 and 3.3V from battery but the 3rd ???
Not all designs connect the power input to a converter. Some of them will power the GSM RF amplifier directly from the li-ion cell, and that one will definitely not withstand up to 20V. It's you cell phone after all, try and see, but don't blame others of smoke comes out.