Greyhound buses used to stop for pick-ups and drop-offs. It's been a few years since I've taken a Greyhound from 95th Street and remember Greyhound personnel perhaps the drivers using the phone. The station where you can buy your tickets was actually in the terminal near the fare controls, however, that changed in the late 90s.
By 1999 at least one of the last few times I took a bus from 95th the actual ticket office was moved to the back where the 29 State bus starts its journey. Sometime after an overhaul which had been in 2002-03 Greyhound began drop-offs and pick-ups at their ticket office in the back of the CTA terminal.
Sometime during the past decade this phone was disassembled and then the line cut. Funny part is that I knew it was there but hadn't thought about it in years until seeing this recently. Whenever CTA finish the new 95th terminal this ancient device will be history.