why a hassle buying door bell ?

1) your hse should supply 5 wires , 2 wire ( from outside switch )
2) 3 wires ( L / N / E )

then

u buy legrand bell(build in transformer) . has 4 connection. 2 input for L/N (polarity does not matters here and it is used to convert 240 to 12V dc ) and 2 input for switch ( for connectivity )

legrand is a brand of door bell.

door bell is not a hassle but it become a hassle for some ppl here