Remember where you are. Ranting down the phone about work is inappropriate on crowded streets and rush-hour trains or in restaurants. It's annoying for others, and your contact at the other end won't appreciate every Tom, Dick and Harriet knowing their business.
Be professional. Don't call from areas with dodgy reception, and warn the other party that you're on a mobile. Control your volume - you may be struggling to hear yourself over the traffic, but to the other person it will feel like you're shouting.
Take it with you. The aim is for colleagues and clients to be able to reach you. A mobile ringing all afternoon on your vacant desk is guaranteed to nark your colleagues - especially if it has a novelty ring-tone.