One thing which might be worth checking is the battery contacts.
I just got a second hand extra phone from ebay.
It didn't work at first and I had to bend the +ve battery (i.e. the contact which isn't a spring) contacts with a screwdriver to get them to make contact with the batteries.
On this phone the plastic battery compartment can keep the battery from making contact with the metal plate.
This might be the solution in some cases.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cordless phones.