The absolute maximum allowed is 120 mA. The max recommended is 60 mA.
Typical values range from 23 mA to 55 mA, with 28 mA to 45 mA being the most common.
Perhaps the most notable thing about the above is that any resistance in series with a loop should be capable of dissipating the heat generated when 120 mA of current flows (and the assumption should be made that enough voltage can exist to cause that much current). That can be a significant amount of heat!
At 20 mA or greater, it is offhook. At 18 mA or lower, it is onhook.
The switch should supply a minimum of 23 mA.
