I have implemented voice-port cable-detect on my VG350 (15.3(2)T) with SM-D-72FXS modules and MGCP signalling loop start. The documentation is not specific as to what is defines "connected" and "not connected" state and I have a functioning phone that shows "not connected" state.
The VG350 performs cable-detection by using low-voltage REN measurement. This means the FXS port will inject a low ring voltage into the line and measure the impedance. If the impedance is low (means there is a ring circuit) the assumption is that there is a phone plugged in.