Doorbell Press for MCU
Providing that your Chime power supply feeding the pushbutton(s) is 12 VAC, I'd suggest a somewhat simpler schematic:







simulate this circuit &ndash; Schematic created using CircuitLab

These current transformers are readily available as an Arduino type sensor for measuring 5A AC full scale. They are ideally suited for sensing presence or absence of AC current and won't interfere with your Chime circuit at all. 

It's likely that your Chime is using something between 2-5W when activated, so you expect current flow at 200mA or above when activated. 
You can pass the pushbutton wire through the current transformer up to say 5 times which would give about 1A full scale from the current transformer. At this full scale setting with only the burden resistor on the sensor board, you should get a nice pulse going into the 'Pi digital pin. 

Search for Arduino AC current sensor on Ebay or Amazon, you should find lots like this: