Thanks for reply. I found the answer. So, for other people who might run into the same problem here's the answer. Use the dip switch config on the USR 5686 modem as follows: 1,4,6,8 up and 2,4,5,7 down. After setting the switches in this config instead of the default I found in the manual for this modem, the "faxaddmodem" was able to probe the modem sucessfully.