Assuming the bell switch is electronic or a relay with low on resistance when forward biased the resistor wattage will vary according to the transformer voltage selected. So assuming likely rms voltages of 8 V, 12 V, 16 V and 24 V, the following dissipations (Rounded up) would be seen in a 25 Ohm and 33 Ohm resistors:
24 V....... 25 Ohm = 24 Watts, 33 Ohm = 18 Watts
16 V....... 25 Ohm = 11 Watts, 33 Ohm = 8 Watts
12 V....... 25 Ohm = 6 Watts, 33 Ohm = 5 Watts
8 V......... 25 Ohm = 3 Watts, 33 Ohm = 2 Watts
RING have rounded the Power dissipation ratings up to industry standard values, ie. 2 Watt and 50 Watt, hence the huge discrepancy between 24 Watts absolute tops calculated and 50 Watts recommended. Of course even with the figures I've calculated, you'll only get the resistors up to the maximum case temperature if you keep the button pressed continuously which obviously won't happen. So, if RING suggest 8V as the lowest voltage we really only need to use a 2 Watt 33 Ohm resistor, likewise if you only have a 24 V output from your transformer, then you only really need a 25 Watt 25 Ohm resistor.
One other thing to remember. Round axial leaded resistors (leads either end) are specified in free air and usually between 25 deg C and 70 deg C. So if your resistor is exposed and is not in an enclosed or insulated box then the power ratings I calculated are correct. If encapsulated, then you'll possibly have to increase the rating. Panel mount resistors dissipate their heat from the base to the mouning panel. So if the Panel is large and at 25 deg C the rating is correct, but if the panel is warm or you aren't bolting it down you'll again have to derate the resistor (Increase the max dissipation figure).