"There is NO WAY I'm going to have anything other that a proper POTS line serving the phone. "
If the Australian model is anything to go by you will not have an option to retain the central battery POTS line - the equipment will just not exist. The Australian NBN (BT Openreach equivalent) propose that in addition to a VoIP line, with local back up battery, you also need an additional 3/4G mobile interface to overcome problems should the intermediate carrier equipment fail. All this adds cost for lift owners / operators and of course assumes you are in a mobile phone coverage area!
A simple description with photos at https://mrtelco.com/blog/nbn-lift-phone-emergency