Err... can all lefties just speak for themselves, please? I'm a leftie, but I wouldn't voluntarily hold the phone in my right hand while making a call unless my left hand was in a cast.
No two lefties are alike in this respect. I know total lefties who inverse absolutely everything, they keep the mouse on the left side of the keyboard and hold a guitar like Jimi Hendrix. Myself I do everything like a rightie except writing and brushing my teeth.
You and I are exactly alike. I'm left handed and NEVER hold the phone with my right hand. Nor do I use the Grip of Doom when holding the phone. And like you, the only things I really do with my left hand are write, brush my teeth, hold the phone, or eat. Everything else (playing the guitar, using a mouse, throwing a football, shooting basketball, etc.), I do with my right hand.