Rodney has it right and the manufacturers agree with him.
The reason most batteries in cordless phones go South is the same as the reason for loss of capacity often erroneously referred to as "MEMORY" . The reason is OVERCHARGING. Overcharging kills many batteries before their time.
An occasional discharge ?charge cycle can help equal out the charge between individual cells in a pack. When cells are discharged the voltage betrween the cells may vary . The weakest cell in the pack will be a large determining factor as to when the PACK is deemed "discharged" Since the voltage drops very quickly in NiCads when they reach 1.0 volt , the lowest may well go BELOW 1.0 Volt while another in the string may still be at 1.18 or so. When recharged ,the lowest cell may not get a full charge and so the vicous cycle goes .....
Some manufacturers reccomend a SLIGHT ,OCCASIONAL overcharge to help bring the lowest cell up to full charge. A slight overcharge at a low charge rate is easily tolerated by modern NiCads. Not to be confused with repeated overcharge or sustained trickle charge (a practice most manufactuers frown upon.

Whirley bird made some comment about matching his cells in home made packs by cycling them befrore soldering them up . This is an excellent practice as matched cells will minimize the effects of the lowest cell in the string.

The people who make the radios don't make the batteries and the myth of memory is an urban legend that has a pretty good hold. It has gone so far that you will often hear of people cycling their lead - acid batteries . This practice will usually do nothing but shorten a lead acid batteries useful life.