Use of "+" sign with a cell phone

Somewhere I read that when dialing from a cell phone to any other phone, you must dial a "+" sign before all other numbers. So, for example, if I had a cell phone with a Chinese SIM card in Xi'an and wanted to dial a fixed phone in Beijing, I would dial the "+" sign and then the city code for Beijing and then the number: +1012345678. Then with a fixed phone the "+" sign is replaced with a "0." However, I cannot find that information again. Do I need to use the "+" sign when dialing from a cell phone, and if so, under what circumstances?

Thank you.