If you have a Bell-shaped head, it makes sense. Consider an old school FX (Foreign eXchange) line. The FXS (Foreign eXchange Station) device is at the customer end and one side connects to a phone. But the other side (connected to the carrier facility) is communicating with the FXO (Foreign eXchange Office) device, i.e. using FXO signalling. Likewise, the FXO box in the CO is communicating with the customer’s box using FXS signalling.
It’s the same ‘logic’ as why we buy termination to make calls and origination to receive them.