But they already do that with warrants. As long as they have a warrant and aren't breaking encryption by putting in backdoors, they should have the ability to listen in on conversations.
Yes, without encryption, they have that possibility, after they prove probable cause. Without getting into a debate whether that is appropriate, it is important to keep in mind that it is the citizen's right to privacy of conversations is the more fundamental here. The government does not have a right to snoop on everyone's conversations, they simply have this possibility in certain situations.
That's how it is now without encryption.