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First, you aren’t going to be able to use the Phone-Jam with a cell phone without some trickery. You have to provide a POTS/analog dial-tone to the Phone-Jam, there’s really no way you can do this from a cell phone without building some complicated circuitry. As far as using a digital line, I’m not clear on what you are trying to do. If you are using the Phone-Jam on a digital line, then yes a digital phone adapter should work ok. Like you’ve read, directly connecting the Phone-Jam to a digital line would be very bad, as long as there is a digital adapter being used though it should be ok. However, connecting a cell phone to the Phone-Jam is a completely different subject than connecting it to a digital phone system.
You MAY be able to wire the cell phones mic and headphone to your playout system’s mic input. The easiest way is to use a cell phone with a phono jack/headset connector. Then make yourself a headset-to-computer cable. Generally a headset plug will be a stereo phono connector with the mic on one channel and the output on the other channel. Usually a phone has both sides of the conversation on the speaker, so all you need is to wire the speaker to the mic input on your computer. You will have to experiment a little, but it shouldn’t be too hard to setup. External mic into the phone, phone output into the mic of the playout system.
Another alternative if you really want to use the Phone-Jam is to setup an Asterisk PBX on a Linux system. Again, this isn’t trivial, but with an Asterisk PBX card you could provide a real POTS dial-tone to the Phone-Jam (and have real phone connectivity via a VoIP provider or interfacing to your digital phone system). This should only put you back ~$200 to build a complete system on the cheap.
Hope that helps some.