802.3af is becoming the world standard regarding Power over Ethernet but Cisco phones are able also to do power inline (also called pre-standard), it was originally the standard developped by them before standardisation.
With pre-standard phone , the Cisco switch is able to see the reflected FLP ( Fast link Pulse) coming back on its port. Then the switch apply the current on the port to power up the device.
With 802.af , the switch (this time, it is more generic) is able to see the current coming back on its port from the Resistive Detection and Classification stage.