Check the telco line at the junction box, remove cover and be sure its clean and tight(snug). Use only the modem to the telco line, no other telco device. Eliminate all devices that area attached at least to verify the modem works alone to the system. Be sure to de-install any other modem s/w, etc., if so installed. Boot into safe modem, find the modem and delete again any found. Once done, reboot and then attach new modem and and any driver s/w and attempt connection. if you use some ISP s/w, reload and repeat the process for the modem removal s/w, etc.. Doo all this at the same time or retry. It important to try as a clean slate as possible. If you pick-up the phone and get a clear dial tone, it should be alright. But, if you hear a scratchy or slightly noisy line, there maybe an issue. Do all that and report results.
I was attempting to fix a connection error for a friend of mine with her 56k modem. I tested several things including: connected to "line" and not the other port, replaced phone line with brand new one, phone gets dial tone no problem, answering machine does not interfere with the dial tone when messages are present. I also plugged in my external USB modem by TrendNet (http://www.newegg.com/Product/Product.aspx?Item=N82E16825134003).
I am connecting intermittently but not 100% of the time like she wants. I also tried changing the COM port the USB modem uses from 5 to 3, 2, and 4. none of these affected anything. She does have an older system (e-machine with XP) but i do not suspect it to be the machine considering i plugged in a brand new modem. She is in an apartment complex and i am wondering if there is just simply too much interference in the line with insufficient RFI shielding?? Any suggestions?