An apparatus for detecting customer premise equipment alerting signals (CAS) in a first telephone having a transmit circuit and a receive circuit and a hybrid line termination for connecting the transmit circuit and the receive circuit to a central office line. The apparatus includes an off-hook detector for producing an off-hook signal identifying whether or not the first telephone is in an off-hook state, an extension-in-use detector for producing an extension-in-use signal identifying whether or not a second telephone connected to the central office line is in an off-hook state, a CAS detector selectively operable to receive the CAS signals directly from the central office line and from the hybrid line termination and a control element for controlling the CAS detector in response to the off-hook signal and the extension-in-use signal. The control element acts to enable the CAS detector to monitor the CAS signals on the central office line directly when the second telephone is in the off-hook state and enable the CAS detector to monitor the CAS signals from the hybrid line termination when the first telephone is in an on-hook state, and the second telephone is in an off-hook state.