Today’s invention is a mobile phone which needs no battery replacement. Instead, the clamshell design is made robust enough to act as a grip exerciser: the kind that squash players use, consisting of two handles which are sprung apart.
The halves of the clamshell would be similarly sprung apart (when not in the closed position). In between calls, a sporty user could repeatedly (and probably ostentatiously) squeeze them together and thus store enough energy to charge the internal battery.
Less sporty users could place the phone on the ground and press on it with their foot a few hundred times, in the manner of someone inflating an air mattress.