Cell Phone to Cox Landline Problem

We have a sporadic but ongoing problem (6+ months) with family & friends who use cell phones to call our Cox landline. Sometimes, our landline phone will ring but when we answer, all we hear is total silence. The caller hears only continued ringing. If they disconnect & call again, our phone does NOT ring & the caller gets a busy signal, sometimes for up to a half hour. These callers are using different kinds of cell phones, different service providers, & are calling from all over - down the street & across the county - so we know the problem is with our Cox landline service & not a particular cell phone service. We've spoken to several (very nice) Cox customer service reps who have tried to fix the problem, but it still continues. Is anyone else having this problem? Has anyone had any luck resolving the issue?