Is it possible to get phone line audio I/O on a computer in a programming environment?

It is, but in order to do so, you'll need to become familiar with your modem driver library. If you can identify which DLL(s) they are, you can look for function names among the binary data and try to find doentation for them online. Sorry I don't have any more detail than that for you. I've worked with modem programming, but not for voice/audio.