How does the Apple Watch make a telephone call if it’s not connected to any telephony services?

If it's not connected through an iPhone nor Cellular services, it can't make a call. But when paired with an iPhone, which has cellular service, as long as the phone and watch are close to each other, it uses the cellular connection from the phone to make and receive a call.

When your Apple Watch does not have a cellular connection, then think of it as just another Bluetooth headset that can tell your iPhone to make a phone call. Or like a car which is paired to an iPhone via Bluetooth, and you have handsfree calling via your Car's audio system.

The added advantage is that the Apple Watch can use WiFi if the iPhone is just out of Bluetooth range, but both of you are on the same WiFi network.