"Phone and Messaging Storage" app at nearly 12 GB of internal storage!

So I posted about this in the past but the one response I received was not helpful, and since then (only a couple of months ago), another 3 GB of storage has been eaten by this mysterious app. I cannot believe that the storing of the few texts I send could add up to 3 GB in just a couple of months, if that's even what this app does or what takes up the memory it eats. Can someone tell me what this app is and does??? It comes stock with the device (blackberry PRIV, marshmallow, 6.0.1) and cannot be disabled. The person who responded the last time just said it could be because of something I downloaded that made it begin to malfunction, but I have no way of knowing which app that is and am avoiding a factory reset, plus still have no idea about the nature of the app itself. I did contact T-Mobile at his additional suggestion to ask them to reflash the software, and they said it would be irrelevant and couldn't do that anyway. My internal storage is busting at the seams, phone consequently isn't functioning properly, and I'm so bewildered by this app.

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