You'll probably want to use the CP (call progress) mode of the 8888 to detect the "busy" tones that sound when the person on the other side hangs up. Set bit 1 in control register A, and monitor the 8888's /INT line, which will tell you if audio is present on the line within a certain frequency band. Use timers to see if the /INT waveform matches the "rhythm" of a busy tone.
Also, some PABX systems use a DC polarity reversal to indicate the connection has ended. You can monitor the current through the telephone line with an opto coupler.

Hans