IMHO the screens on a cell phone are way to small to be looking at while driving. Most don't have all the options that a GPS will offer either. And if you want to use the phone there goes your routing info while you are driving. Not to mention how fast it will eat the battery!! Get a new GPS.
I have a GPS that I can talk to it to plan a route no more punching in individual letters of a road and city to get my route planned. And a real large screen area that can be seen from the dash. One of the drawbacks of using a cell phone. I've yet to see a cell phone that will tell you your speed, alerts you if you are going over and what the speed limit is where you are at in real time, among all the other options a 'good' GPS offers. Just saying, JMHO :C