Hi,

It seems to me there is no limit to what current you can draw while
off-hook except that the current is limited by the CO, 26mA is a minimum
available at 6V.  On-hook is a different story where the limit is very low,
1uA except while ringing and then ?

(numbers from The Art of Electronics, Horowitz & Hill. 60 US$ you'll never
regret spending)

Jim Hartmann