Free Dial testing software available

[countryman]

The big advantage of the COM port is that it supplies enough power to put a "real" DC load on the dial contacts. Also it has a number of control pins in addition to the 2-wire data line, which allows to measure pulse and shunt contacts at the same time with no need to build further circuitry for the purpose.
Adapters as mentioned in this thread are still available, work with modern device and are absolutely worth the expense if it comes to reliable and easy dial testing.

The mic connector of laptops etc. supplies a tiny DC voltage and current for typical electret microphones, but basically is an AC only input. The switch status appears distorted (sawtooth signal) at the sound card, still allowing to measure switching time but not a real DC measurement.