Basically your question is beyond the scope of the forum ; e.g., Admin, User Guide, Programming help.
[I am assuming that the phone in question is out of warranty.]
Do you have the KX-DT343 Quick Reference Guide? Page 17 shows how to install the Wall Mount...so, at this point the back is partially disassembled and at that point the screws can be removed. (I don't have a phone available in front of me
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So, I deduce that if you want to replace the keypad and button pack it's because someone spilled a sickly sweet sugary soft drink on the phone?