I've often done this, assuming that it's the same situation as looking at a map while driving. But what I could find about the topic is that using the phone "in any other way" (than communicating) is prohibited. To be sure, I'm usually not driving at speed looking at the map, rather consulting it while stopped at lights or the like. But might I get pinged by a camera looking at the phone?
It seems to me the ban does not consider this obvious use of a phone. (I didn't need the law to stop me from texting, calling, or reading anything while driving.)