convert USB to RCA phone plug in order to close a switch
There are devices available such as this one (also available ready-built) that will do what you want. That might be bigger and more cumbersome than what you're looking for, however. 

In reply to comment: Not much simpler, for USB, although you may find something smaller, with fewer channels - just search for 'USB Relay'. This looks the most promising route.

An alternative approach, if you do want something simpler, then you may have to step outside of Windows - you don't say anything more specific than 'software' but I'm assuming you mean Windows or anyway a modern OS - but in the days of DOS, you had more direct access to the hardware, so could for instance use one of the RS232 control signals (DTR etc) plus a diode and a small relay. But AFAIK this isn't available under Windows. In fact - ref comments - this is still available under Windows too.