So around 9pm last night, the cell phone I'd found on the sidewalk near Quaker Tower rang again. This time the called ID said "Linda T-" and the caller was Linda, the phone's real owner. She works right around the corner from where I found the phone and I gave her instructions for picking it up at my office. I also offered to mail it to her, but she said she didn't think she could survive that long without her telephone.
She called me a couple of times this morning, once inviting me to join her and her friends at Klay Oven, an Indian restaurant around the corner from my building. Eventually, around noon, she called again and I met her downstairs in the building lobby. She was a little heavy and frumpy and probably 40ish, which isn't surprising as Lindas tend to belong to a single generation.
She asked for my business card, which I gave her. My boss thinks she'll send me a thank you note or gift, but I'm just glad the mystery is solved, a happy woman has her phone back, and I've made my contribution to karma.