This is true for Ni-Cd batteries and to limited extent Ni-Mh cells. You may want to verify the cordless phone was one of above chenistries. You do have to discharge them all the way so they every so often " remember" their full capacity.this is called memory effect.Li ion cells don’t have this effect.
Even then if they are always kept fully charged electrolyte and electrode degradation has an Arrhenius relationship wrt to voltage AND temp. In other words high voltage = more degradation. So better to use battery after it is charged or at least stop charging. But useing it all the way down - I would not recommend it. Cycle life decreases EXPONENTIALLY with depth of discharge (DOD).