I've read that battery calibration apps are only a placebo effect and they do nothing. Is this true? What to do if your phone suddenly shuts down at 25%?
Battery calibration being a myth was asked here Bought a new battery, do I need to calibrate it? - incidentally by you :-), so yes to repeat, it's a myth (see Izzy's answer that stressed this)
To address shutting down at 25%:
If the battery is old or heavily used I would first change the battery
Check if the behavior persists in safe mode. If it does go to next step. If it doesn't it's caused by some app you installed (unlikely)
Do factory-reset. If it continues, flash stock ROM (updated if available) or custom ROM
If none of these help head to service center get an estimate and choose between repair or replacement of the device