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