"anyone" can not listen to your cell calls. Only people that have access to inject commands into the SS7 network that your call is routed through can do that.
Yes, but numerous people participating in the Osmocom projects have that expertise. Fortunately, they're interested in building an open-source baseband processor (among other cool things), and not in hacking into anyone's private communications.
Besides, base stations are available openly on the market now at pretty reasonable prices. It's why I never talk about anything truly private on a mobile phone.