Ask HN: Why landline make crazy noise while hung up?

I decided to ask it here because the problem to me at least is so novel that I found it curious and interesting (although I still need to fix it...)

Basically, one of my landline phones on the company decided to make ear-splitting loud noises... but only while the phone is not in use, when you pick the receiver up the noises stop.

I have no clue what could do that, I didn't knew a phone could be that loud (you can hear it across the room!) neither that it could make sounds while not in use.