It does if they admit to a crime or the planning of one.

If somebody is stupid enough to admit to a crime they did or planning, and there is evidence that leads to arrest, they are fucked. This isn't like TV. You don't need a warrant. If you are in the middle of a divorce you can record any phone call with your soon-to-be-ex as they are considered your wife still and they don't have reasonable expectation of privacy.

You only really need permission if you are archiving the conversation and there is no other reason to expect otherwise. If they are stupid enough to not agree to recording, and they admit they murdered somebody, they can still get arrested.

This would be completely admissible as evidence as long as they can identify who/what/where/when/why.