I used 4P4C (so-called "RJ-9") jacks, which was a big mistake, because they're much harder to find, and the associated cables are not commonly available in junk boxes. 4P4C is what is commonly used for handset cords (the curly ones). I updated the design to use 6P4C (so-called RJ-11/14/25) jacks (the commonly-used wall-to-phone cable), and also made the board Gadgeteer compliant, and the result is much more satisfactory. There's also a footprint for a DS18B20 temperature sensor onboard. This solves the most common use case all in one solution.
We've had a bit of an issue, however, because the new revision of the board didn't actually work... I believe it was because the connector was backwards, but we've yet to verify that. More updates as they happen.