English to Chinese Tech/Engineering IT (Information Technology) VoIP

If it is, the next step is to do a full network inventory. You can't decide what you need until you know what you have, and too few organizations, says Pierce, actually know what they have. Not only should an inventory count routers and switches, but Pierce says that it should include an assessment of how much traffic is carried on-net and how much is carried off-net, bearing in mind, she says, that "on-net PSTN (public switched telephone network) minutes are the cheapest thing on God's green earth."
问题针对最后一句：on-net PSTN (public switched telephone network) minutes are the cheapest thing on God's green earth.
不是网络电话才是最廉价的通讯方式吗？怎么会是通过传统PSTN的通讯方式呢？