A lot of these cute little phones (mine included) seem to suffer from
progressive key failures on the joystick mini-keypad.

Determined to fix the wretched thing, and after acquiring the
necessary Torx t4/t6 screwdrivers and the wonderful detailed
disassembly guide - search

"how to change sl56 housing"

on google (I think the SL56 is almost identical, AFAIK)

I found that the hardest bit is to get the slide off. Short of poking
four screwdrivers through the little holes at once, I found that a
plastic comb, suitably mangled to leave only four teeth on the
comb,spaced appropriately, worked extremely well as you could then
press down and disengage the slider detents much more easily.

When I looked at the way the mini keypad works, I can see why problems
occur. The base PCB has a set of concentric circular contacts which
are bridged by metal domes, with a click action. The domes,
astonishingly, are merely attached to an adhesive plastic sheet which
is positioned over the PCB.

This is quite different from the normal rubberised conductive assembly
you usually see, and there's a serious design problem here, because
the tolerances are very tight, and under use the adhesive tends to
'sheer creep' which will pull the top layer out of alignment.

At this point keys start failing one by one as the alignment causes
the dome assembly to fail to concentrically match up with the
underlying contacts.

Carefully peeling back the adhesive film, taking care not to stretch
it, I used foam rubber Q-tips - NOT cotton buds - to clean the gold
pads and then I lightly moistened a clean sheet of A4 paper with
contact cleaner, and gently rubbed the domed side of the film across
this, taking care not to attach the adhesive film onto the paper.

To reassemble to the tight tolerances requires a good bright desk
lamp. Shining this directly down, position the film layer and without
pressing it down, move it around until you can see an even gold 'halo'
around each of the domes. Then press down and ensure the film is
firmly reattached, with all domes centrally positioned.

PS: don't be tempted to do what I did before I knew how to disassemble
the unit and tweak the tiny little philips screw which is underneath
the top PCB (that the screen and mini keypad are attached to. This
appears to be a contrast adjustment for the screen. I had to pull the
phone partially apart and readjust this screw due to my own stupidity.