Sorry guys, I've been too busy...
anyways, here is the scoop:

Setting 'Loop Current Disconnect Duration' to 200-300 ms seems to have fixed my problem with the answering machine recording the busy tone; however, the answering machine was able to record a short (10-15 ms) dial tone (just right after the callers left their messages and hung up). I personally tested this condition via both cell and regular POTS phones

Setting 'Loop Current Disconnect Duration' to 3000ms did not seem to cause any problem nor improve anything

FWIW, I did not remember what I did to the SPA-1001 that fixed the very same problem when I had ViaTalk, but I'm glad that the situation has tremendously improved

Cheers,
-v