Kevin's right, just revisited '25 yrs'.
If the 'sliding' does not work, look for a tiny notch, somewhere around the side of the (prob. avocado green or powder puff blue) colored phone case plastic cover.
Disconnect the hand-set from the side to get it out of the way, then gently use a...
Try grabling the phone unit and pushing straight back against the wall, then sliding up.
There is often a metal plate back there that has 2 screws, the back of the phone has 2 holes. By sliding the phone up hard, you are sliding the 2 holes down a tiny track into a larger area were the screws...