Dial up, yes dial up

I went to a job the other day and this persons dial up was not working. The computer said when tryingto dial that it either had no dial tone or that the phone line was already in use. This was connected to a second phone line. There was no phone connected from the pc to the sencond phone outlet. They said that they have always had it like that and it works. The went on a weekend trip and came back and it has not worked since. The computer says it has 100% communication with the modem and the device mananger shows no problems. So I am stumped.