Why won't my phototransistor interrupt my microcontroller?
Short answer, the 0.8v is probably not low enough to count as "low" logic level. The specification for this (Vil) is explained in the microcontroller datasheet.

As an easy solution, add a schmitt-trigger input device, such as a 7414 gate between the phototransistor and interrupt input. This will ensure the microcontroller sees only Vdd or Vss, and it provides some hysteresis.