I'm sorry you hadn't received an earlier answer. Various Experts are online at different times and I recently signed in and was notified of your question.
I'm sorry you're having this inappropriate elimination problem with Phoebe. The first step is to have her evaluated by her vet, with a urinalysis, urine culture, and fecal analysis, to make sure an underlying medical problem is not causing this behavior.
However, after reading your information re: her litterbox being in your old room, plus that room being her 'time out' room, I do think this has a lot to do with the problem. First, I would suggest that her litterbox is located elsewhere and another box is added in a different location. Make sure the box is not covered and the litter is unscented. You can add 'Cat Attract' litter additive to each box; it includes herbal attractants that will encourage her to use the litterbox. Here is more information:
This is usually available in pet supply stores and others, like Walmart, or you can order online.
Make sure to clean all areas where she's soiled with an enzyme-based cleaner for pet stains/odors. Some well recommended ones are Nature's Miracle, Zero Odor, Odor Mute and many others found in your pet supply stores. When the scent is completely gone, she won't be encouraged to eliminate in those areas, again.
I hope all will be well with Phoebe!
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