The line is busy – modem problem?

I installed a Harmony 56K modem (comes up as Agere PCI) in my customer’s ME computer. It appears to be working fine. But when I try to dial the ISP, the modem gives me an error message that the line is busy. This happens even before it finishes dialing the number (when I was using 9,1,). Then I got rid of the 9 and 1. Then it told me the computer did not answer. I called the ISP connection number from my phone and got through fine. What’s going on? I should mention that the customer originally had another modem in the computer and they were getting no dialtone errors.