The easy solution was to toss them all and buy identical new ones. So we bought a Panasonic Expandable set, with a base and three handsets. When we converted to VoIP it was easy to plug in the base, and keep going.
I regret to inform you that our trusty Panasonic DIED on us. It was slow and painful. Sunday night the base wouldn't quit beeping, but we could still make calls. By today it had flat-lined. We will miss the old joker. He gave us 13 years of service. The good news is that we set up our Vonage service to automatically transfer an unanswered call to my cellphone. The caller on the other en never notices the difference.
I wonder if I can sell the handsets on E-bay?