If for no other reason, landlines are good because he gets 911 emergency. They're also carrying their own power, so in the event of a disaster, he can still make calls. There is a thing called e911 that is through a VOIP service or cellphone, but it's not nearly as accurate as the real 911 is. Also, if the internet is down, it won't work anyway.
Since Don is running a theater business, he's going to want the security that comes with a landline.