In 1996, after a decade spent bringing the Disk Operating System parts of GLaDOS to a state of more or less basic functionality, work begins on the Genetic Lifeform component. During that time, the Aperture Science Red Phone plan is implemented in case GLaDOS appears to become sentient and godlike, requiring an employee to sit by a red phone on a desk in GLaDOS chamber's entrance hall.
Not only does the old-school twin-bell ringer work FLAWLESSLY, but the handset is Bluetooth so it interfaces with my cell phone, which also has the old-school twin-bell ringtone! I disabled the phone from sensing when the handset is picked up, and from dialing out or picking up calls due to the Bluetooth handset, I just have the ringer enabled. I love hacking 30 year old electronic technology
Be sure to check out my gallery for other awesome Aperture-related stuff! See the phone in my Aperture Office here: [link]