We had exactly the same problem with two of ours, took them apart, did all
the cleaning process and reassembled carefully - didn't make the slightest
difference and put it down to bad design.
My wife had a brilliant idea and purchased another set of Philips ones to
replace them !!! - lasted about16 months until they too had exactly the
same problem and were swiftly binned.
Next up were a set of Panasonics, the buttons are fine after just over a
year but the internal speaker on both units failed within 2 weeks of each
other - one is being fixed under guarantee but the other was 1 week over a
year old, Argos wouldn't honour it but Panasonic relented - due back soon
but not holding my breath.
Back to the 2 cans and string I think.

LOL, I'll stick with my cheapo Binatone DECT phone thanks - 3 years old,
been dropped, thrown at the wall, left in a hot steam-filled kitchen, and
still works perfectly. The only thing I've changed is the NiCd batteries,
everything else is as good as new

Hellraiser............