Knock Three-One-Two by Fredric Brown

Ruth Fleck had not yet left the restaurant. George had told her to go at eleven-thirty, but the last customer, at the counter, had obviously been within a minute or two of finishing and she'd decided to wait. It had paid off, too, with a two-bit tip that he probably wouldn't have left if he'd seen her leave; he wouldn't have known that George would hold the tip for her and give it to her tomorrow evening.

She'd carried his dishes back and was putting on the light summer coat over her uniform dress when she heard the phone ring up front. She didn't hurry because George was up there starting to check the cash register, and anyway the call was unlikely to be for her. Nobody she knew would be calling her at this hour except possibly Ray -- and if he looked at his watch before calling he'd think that she'd already left.