Almost all cordless tools are powered by rechargeable batteries of the Nickel-Cadmium type. In typical use, the phone is only used for a short time before it is put back on its cradle for recharging. Some cordless phones have low-battery indicators, so you are not going to be cut off. Very few cordless tools use standard size batteries, so they are more expensive to replace. The cadmium in the battery is also a poisonous heavy metal, make sure to give the old battery to a hazardous waste recycler, if possible.
Many communities have annual hazardous waste collection programs.