How should a cell phone number be if you want to call from one country to another, like from Utah to Portugal?

What is a good example cell phone number if you want to call someone, lets say from Utah to Portugal? And do you have to put the (+) when you dial the number? (Example: +351 + __ + __ + ___) ??? I'm trying to make a long distance call, but I think the numbers I'm using are inaccurate. Everything I've tried doesn't seem to work, so what am I doing wrong? Could someone please help me?