I’ve seen Panasonic cordless phone sets at the thrift store many times. There are usually several handsets and an answering machine base unit in a bag for $5 or $10.
Rather than fiddling with the old-style battery packs, perhaps it’d be easier to check your local craigslist or Goodwill stores for a gently used set that takes AAA cells. Swap the old cells with IKEA, Eneloop, or Amazon Basics NiMH.