BCM V.92 56K Modem Won't Connect

I have a brand new Inspiron 8600 Laptop with a BCM V.92 56k modem and it won't connect to the internet through my ISP which is AT&T. I get an error message "678" - loosely defined as "No Connection". It dials fine and makes all sorts of noises and eventually disconnects and tries the backup number with the same results. I downloaded the newest AT&T software on my regular home computer and ported it over to the laptop. It took several tries after installing it for it to get through to AT&T and complete the account registration process on a new computer. At the beginning of the whole process, I was able to download Dell's list of ISP providers, so I know the modem is working. AT&T is our ISP for two other computers in our house and runs as fine as dial-up is going to in a rural area. Our connect speed is between 19.2 and 24 kbps on a good day. I've been all over Dell's and AT&T's help pages but to no avail. I've reduced the "Max Port Speed" to 57600 and tried such recommended strings as "AT+MS=V90" to no avail. Quite frustrating. The modem driver is version 3.5.25.0, dated August '03 and my operating system is XP Home Edition. Any help would be most appreciated.