I looked at quite a few programs of this type and decided on IVM because it was stable and reliable. A few features don't seem to work at the moment (on-screen caller ID of incoming calls). Others one might want are missing (e.g., a telephone ring through the speakers for non-blocked calls, a fast/slow flashing keyboard light for new/old messages, etc.). However, the program and help is very straightforward and I haven't purchased technical support yet.
I'd recommend trying it out, at least for a few days, before buying so that you're at least aware of which parts work and which do not.
I'd also recommend to set it to "Run as Service (before login). Show this user only." (This user is whichever logs in first.) Additional users should access messages using the web server feature and NOT launch a separate instance of ivm.exe since this will indeed break the answering machine functionality. (This seems true for all programs like IVM as well.) The setting "Run as Service (before login). Show all users" launches an instance of ivm.exe for all users. If you use fast-user switching, this is a problem the moment a second user logs into the machine.
I don't know about the board, but the sound quality is really good, even with the voice modem that came with my computer.