Manuals here:
https://www.bt.com/help/user-guides/phones/landline-phones/cordless-phones-with-answer-machine/bt-freestyle-750
Have you bought these brand new or second hand?
If new, there is probably a plastic tag in the battery compartment that you have to remove (see the manual) before the phone will charge.
If secondhand, check the batteries are OK, not leaking/corroded and are in the right way round.
The batteries may simply be knackered - nicd batteries do deteriorate, especially if left to run flat and not charged. You'll need 2 off AAA NiCd batteries per phone.
Maybe try a pair of batteries to see. eBay or Amazon are probay a good place to start.
Eodjh
October 2021
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