Voice over Internet Protocol (VoIP) has become an increasingly popular way to provide phone services. At this moment there is a transition going on from delivering voice over the Plain Old Telephone System (POTS) to using VoIP technology. Line echo is created in the 2-wire to 4-wire hybrid circuit between the VoIP phone and the POTS phone and is much more apparent due to the large delay in the packet network; line echo cancellation is necessary to ensure satisfactory Quality of Service (QoS). It turns out that the performance limit of the amount of echo that can be cancelled is set by the non-linear portion of the signal. Line probing is a method of inserting a known signal at the far-end and recording the near-end signal. The two signals are then analyzed together for various impediments like, non-linearities, bad ERL, and noise. Line probing is used in this thesis to find the performance limit of the echo canceller as well as other useful metrics of performance.