Found it! It's "Philomel Cottage" by Agatha Christie!!
I just dug it up at my parents' house, it was a story in a Longman simplified edition called "Stories of Detection and Mystery". Had apparently confused it with Poe's Tales of Mystery and Imagination!
"I'm just going to telephone to the butcher," she announced nonchalantly. "Don't you bother to move." [...]
And then, as she nervously depressed and released the little key in the receiver she was holding, which permits the voice to be heard or not heard at the other end, another plan flashed into her head.
"It will be difficult," she thought. "It means keeping my head, and thinking of the right words, and not faltering for a moment, but I believe I could do it. I must do it." [...]
Alix drew a deep breath. Then she depressed the key firmly and spoke. "Mrs. Martin speaking - from Philomel Cottage. Please come (she released the key) tomorrow morning with six nice veal cutlets (she released the key again) It's very important (she released the key) Thank you so much, Mr. Hexworthy: you don't mind my ringing you up so late, I hope, but those veal cutlets are really a matter of (she depressed the key again) life or death (she released it) Very well - tomorrow morning - (she depressed it) as soon as possible."