For a total investment of $29.00 ($20 for the initial purchase, and $9 for a replacement memory battery), I've restored an $8,000 or so late-80's Telephone Network Simulator (essentially a mini-CO in a desktop package) to full functionality. This thing is built like a tank, and was made by TAS (now owned by Spirent Communications), their model 114.
The idea is that it simulates all the functionality of an analog (POTS) phone line. It'll generate dial tone, ringback tone, ringing voltage to the destination port, etc. In fact, to get to the destination port, you need to dial its number from the source port just as if you were connected to a live line. The unit can also generate impairments (noise, reduced levels, line imbalance, etc.) to 'stress-test' whatever you've got hooked to it. Even neater is that you can remote control it via RS232 or GPIB, so it can be hitched to an appropriately-programmed computer for automated testing.
In the context of my lab, it'll be a great unit for line-testing cordless phones, answering machines, and other such goodies, and for proving what a crappy job some of the Taiwan import "fashion" phones do of handling marginal lines vs. older Western Electric (original Bell System) hardware.
Where did I find this cool gem, you may ask? At a ham radio/electronics swap meet, in the Bay Area. Who says such events are "dead?"