A cable modem includes a subscriber line interface circuit (SLIC) that is connected to a telephone. The cable modem can send a signal to the telephone through the SLIC, and detect signal reflection of the telephone in the SLIC. The cable modem measures frequencies of the signal reflection and the output of the telephone. The cable modem also compares the measured frequencies with corresponding thresholds and self-determines to be qualified or unqualified, based upon the comparisons.