Can I use a Voice 56k Modem as a POTS interface to an Asterisk system? If no, why not?
My understanding is that you can, but it doesn't really work well, specifically call quality is lousy. I have used both an SPA 400 which looks to run around $300 on ebay, and an 8 port digium board. Both work very well. Single port digium board looks to run around $30 on ebay, but your mileage may vary. If you are new to Asterisk and/or VOIP, I would reccommend you start off with Elastix, it's a prebuilt system that works quite well for running asterisk on.
A few can, but usually no. The original X100P from digium was just a generic data/fax/voice v.92 modem card. So assuming the modem you have supports that same chipset, it should theoretically work. Note, however, that the modem has to support full-duplex audio, which cheaper ones generally do not. Further, your audio quality may be a bit lacking. But it's certainly good enough for testing and evaluation.