I used the catch-a-call device for about six months and it worked, sometimes but was also a pain. Often it would ring and there would not be anyone there, I believe this was caused when someone was trying to get into my computer. Sometimes zone-alarm would get 50 -100 hits a day and usually this would trigger the device to ring. (very annoying) Normally I could stay on-line as long as I hung up within 20-30 seconds.
Prior to Catch-a-call I used call-wave and that program was a delight. When a call comes through the caller leaves a message and then I return the calls that I want. Had to disconnect first though. Very reasonable but must set up call forwarding.
Currently I have my modem set to kick me off when a call comes through. Not a problem for me as I just reconnect when I'm through with the call. My current modem is a v92 and was hoping to do what you want to do. Unsuccessful so far. Tried two other Isp's to no avail.
Good Luck
jm100dm