This article from NPR addresses the issue. Such a dizzying array of features present themselves to today's smartphone buyer that a simple question like "How's the voice quality?" can be drowned out by the size of the SD card, the apps that come preloaded, and the number of megapixels on the camera.
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According to the NPR article, "Consumer Reports gave most smart phones 'fair' voice quality ratings in its latest reviews. [...] That's in stark contrast to the "excellent" ratings given to display, Web browsing and even battery life features on smart phones."
So what's to blame for the lack of good voice quality on smartphones when manufacturers pay careful attention to things such as battery life and screen resolution?
There seem to be a few factors, according to the article: less room in the device itself for a good speaker; texting and video phone use on the rise; and the fact that cellphones rely on carrier coverage beyond what's built into the device.
A J.D. Power and Associates report linked to in the article has some specifics on particular carriers and how they rank in geographic regions of the US. For example, Verizon Wireless is highest in my area, the Southwest, in terms of "fewer customer-reported problems with dropped calls, initial connections and interference, compared with the regional averages."
It's always wise to research not only the device but the carrier before committing to a phone or a contract. How much emphasis do you put on voice quality when shopping for a phone?