There isn't really a theoretical limit to how small a fusion reactor can be, although clearly this would not achievable for a long time.
Black holes can hold charge, and thus electromagnetism works on them.
(Not really useful for a phone, of course. But manipulating black holes like that is not any harder than manipulating an equally massive lump of ordinary matter.)
> Therefore, a black hole needs to be at least 9.49 × 10^{22} kilograms in mass to emit 100 watts of Hawking radiation. This is about the mass of the Moon.
No clue whether those numbers check out.
EDIT: poking bard a bit more gives this correction:
> However, for a black hole to emit 100W in gamma rays, it would need to be very small, with a mass of only about 10^22 kilograms. This is much smaller than the Moon, which has a mass of about 10^25 kilograms.
But poking more and more makes me less and less confident, because bard seems to believe that bigger black holes emit more than smaller black holes.