A lawful order from a court is reason enough to listen in. It is not unreasonable for telecommunications companies to build in infrastructure to do this sort of thing.
Its those occasions when police or government listen in WITHOUT a court order that bother me. The people who do that need to be prosecuted. Personally.
That said, if you really care about the confidentiality of your conversations, personally exchange a public key with your colleagues and then have your conversation on trusted computing hardware.
It is possible. But it’s not easy.