My Panasonic KX-TGA520M expansion handset (wireless 5.8GHz Gigarange Supreme) suddenly cuts off without warning from time to time when in the middle of a call. However, the battery indicator displays that the phone is adequately charged. Do I need to replace the battery or the phone? I've had the handset for about 5 years.

Yes,it's the battery.Ours does the same thing.Let it charge all day,5 minute phone call,call gets cut off.Being a 5.8 GHz,you may be able to find battery(ies) for it.Mine is older and batteries are discontinued(2.8GHz).Make sure it is rated the same,3.6V,600mAh,or whatever yours is,that is just a sample.Also,it should still be NiMH.Nickel-metal hydride.The packaging wil list the brand names it is for but the numbers definitely need to match 100% or it could cause more serious problems such as battery leaking from wrong rating.The newer technology is DECT 6.0 which was adopted from Europe technology.It is supposed to have better range away from the base,clearer sound,maybe a couple other minor things but I have my doubts as seen on trouble forums and sites such as this one.For one,they work at 1.9 GHz,which will give better range anyway because of the lower frequency used.1.9 GHz is about equivelant to trying to bend a 1/4" steel pipe around a corner while 5.8GHz is like trying to bend a 3/4" piece of pipe around a corner.The 1.9 signal has more "bend" allowed in literal terms.That's the best way I can explain it.Anyhow,I am waiting for a really good sale on phones for holidays to give it a try.Just unwilling to pay full price on a similar technology that was typical of all phones 10 years ago.If you do find batteries,I was going to say maybe buy 2 for each handset,but batteries do not do well sitting around for 5 years.They COULD leak and be worthless by then,unsure but never know.There are many factors to storing them so that they will last.They will be unavailable by then so we can't win for losing.Something like that.Good luck,Greg Also,if you ever get battery acid on you or anywhere you don't want,put baking soda on it ASAP.Baking soda neutralizes acid.Cover in about 1/4" thick layer.