Such devices exist. On a recent episode of the Retrocomputing Roundtable podcast they discussed a device you could buy on eBay for about 30 dollars that would simulate a phone system/network.
It was originally sold for offices and businesses that wanted to set up an internal phone system. It provides power to the phones, generates a dial tone, phone numbers for each phone on the system, etc.
It's like a mini-telephone switching system.
Sorry, I don't remember what the device was called. Have a look on the podcast's website for a mention of it (or listen to the podcast's "ebay" feature).
With this device, you could take two computers with telephone modems in the same house and have them call each other.
EDIT: I found the device. It's called an "Open House H650 Integrated Voice Network"
http://www.ebay.com/itm/380660700507