Regular walkie talkies + VOX + headset re-assembled into the phone handset ? Make the phone hook a switch to cut the mic (err, like it originally did, I guess).

Regular walkie talkies + VOX + headset re-assembled into the phone handset ? Make the phone hook a switch to cut the mic (err, like it originally did, I guess). It's not elegant but it could be a quick evening's work.