A 1948 monochrime oldie (and pretty goodie) from 1948.
Barbara Stanwyck plays Leona, a rich and spoiled heiress. She's also very ill: any attempt to get out of bed exhausts her, and leaving her stately mansion is out of the question. Which would be fine, except that (due to some glitch in the late-40's New York telephone system), she gets patched into a conversation between two lowlifes plotting a murder.
Left alone at home, her only link to the outside world is that phone; we're invited along as she uses it to figure out the target of the plot and tries to convince anyone to assist in thwarting the scheme.
The movie is mostly a melodrama. Told in many flashbacks is Leona's domineering romance with and marriage to Henry—Burt Lancaster!—a small-town poor boy who finds himself, with increasing frustration, firmly under the thumb of his wife and her even more oppressive father.
It's a lot of fun, and it's a real good argument for having a Glock in your nightstand.