Running a numbers station out of a phone number seems odd. With radio it's hard to detect who is listening. With a telephone number it should be much easier to figure out who is calling the number.

True. I'm guessing there's some unknown circumstance that makes the ease-of-access benefit outweigh any security risks.

It wouldn't surprise me if the number was leaked on purpose. To get people calling the station, to obfuscate the message's actual recipients.

I also assumed the intentional leak at first. But the update says the number was then discontinued. Having lots of people calling from other places than the one your spy is in doesn't help.