Leo says that Plantronics has a hybrid headset that will switch between bluetooth and the land line, but there are plenty of options.
Another option is Obihai. It links to Google Voice and it'll ring his phone when someone calls his Google Voice number. It's dirt cheap at $37 and there's no monthly service. This doesn't have 911 service though, so it's probably still a good idea to have a landline that has that in addition.