Dial up connection problem?

[Jennifer]

My grandma and aunt have a dial up internet connection at their house. When the telephone line from the computer is plugged into the phone jack in the wall, it causes the phone line in the house to go dead. If I pick up their phone, silence. If I unplug the same telephone line, the phone has a dial tone as normal. To those who may be thinking this, I understand they aren't going to be using the phone whilst on the internet, but they have to have a dial tone for the internet to be able to connect. It has only started recently, when they were given a different computer by a relative (I don't remember what windows version the old one was, but it was very old--their current one is Windows XP). This is the exact same setup as on their previous computer, and it worked just fine. Why is the line going dead and how do I fix it?

Let me know if you need more details or if I've worded the question in a confusing manner. Thanks in advance.

[Ephemerist]

The typical troubleshooting steps are to first try the modem in another PC, and to try a know working modem in that PC.

Another typical trick is to delete the modem driver via device manager, reboot, and let Win reinstall it.

You didn't mention if it's an internal or external model and that opens a whole 'nother can of stinky worms, but the same 'swap' and 'delete driver' troubleshooting techniques still hold.

BTW XP is 'going dark' on 4/18/14 and there will be no new security updates, so it's a risk to keep using it.