Our Prudent Friend
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Listen to all the phone calls in the phoneboxes
This does not mean find all the phone boxes - just listen to all the calls. There are 12 unique calls in total.
You can do this by resting for 24 hours and revisiting the same phone box -> rinse and repeat until you get them all.
The phoneboxes are only in the garden districts; Barrow Holm, Lud’s Holm and Eel Pie Holm