convert 5v AC to 3.3 DC
I think using a full-bridge rectifier is a bit too trusting. I don't like connecting digital pins to mains, even through a transformer. A transient fault could obliterate your RPi.

Here's my approach - use an optoisolator to provide an isolation barrier. The diode inside the optoisolator conveniently doubles as a half-wave rectifier. I paralleled a capacitor with the ballast resistor on the optoisolator output. This forms an envelope detector.



Here's what the voltage at the output of optoisolator looks like. I had the AC supply on for half a second, then it switched off:



This would probably be fine to feed into the Pi but I personally didn't care for the ripple, so I added a comparator midway between 3V3 and ground. This is what the 3V3_BELL_DETECT output looks like alongside the input:



LTSpice sim is here if you want it.