Dialup on 2009 iMac

I'm going to see a family friend who's been having trouble with her decade old Windows machine. I have an old iMac laying around and I thought I would set her up with it. The only thing is, she lives in such a rural place that the only Internet connection available is dial-up. How can I connect this iMac to the Internet using dial-up? I know there's an option in System Preferences -> Network for the internal modem but do I need an RJ-11 to USB converter or can I just plug the RJ-11 into the RJ-45 port?