I would tend to make a distinction between a holster and a carrying case. A holster would have to be designed to be worn externally and clip on, or by some other means hang on your person, usually from a belt (also sometimes called a belt clip). A carrying case is more generic and could be simply a protective cover you use when placing it in your pocket or purse. Carrying cases would generally include holsters, I think.

iPhone holsters on Amazon.com

iPhone carrying case on Amazon.com (NB includes some holsters)