In the first of these little Audacity tutorials, I'm going to guide you through installing a plug in. In this case installing a telephone filter.
1. First we'll need to download the dll plug in file. I used Telephone voice dialed up from here.
2. Next navigate to your Audacity plug ins folder, it will be somewhere like C:\Program Files (x86)\Audacity\Plug-Ins
3. Place the telephone.dll file into that folder.