[MarkusWandel]

The phone batteries weren't a high load kind of affair. They merely needed to change the varying resistance of the carbon microphone into an audio voltage - on the order of milliwatts of power - to send down the line. A more modern phone, still using a carbon microphone but powered by the line, needed about 20mA of loop current to do this. The telephone terms for the old system vs. the newer is "local" vs. "common" battery.

Heavy duty batteries - specifically the "A" batteries that powered the vacuum tube heaters in early radios - were made rechargeable to save cost.

[VBprogrammer]

This might be a bit of a tangent but I couldn't help but wonder if the appearance of 20ma here is related to the old fashioned, but I understand commonly used, 4-20ma current loop signalling in industrial applications.

[throwway120385]

It's almost never a coincidence. Before digital switching everything was done mechanically, and before mechanical switching everything was done by people with plugs. If you have a big enough industry like telephone switching equipment then you're bound to see a lot of suppliers expand their market by selling the same parts outside of their home industry. Current flow is a nice signalling mechanism because you can tell the difference between short, open, and functional circuit. So I'm guessing it got used in telephone switching equipment and then preserved because there was no reason to change.