The easiest way to do this achievement is to find a phone box which is near an abandoned house, or a house that is easily accessible and has a bed to sleep on.
In the clip below I am on the Main Street in Lud’s Holm with a house around the corner from the phone box I am using. Phone boxes are available in any of the ‘garden’ districts: Barrow Holm, Lud’s Holm and Eel Pie Holm.
**Bear in mind that maps are procedurally generated meaning that no 2 are the same**, so you may have to look around to find a similar setup. Lud’s Holm seems to be the best place for this though. The video will give you more of an idea of what to look for.
What you will need to do is sleep for 24 hours, then go back to the phone box and it should ring, giving you a cryptic message. Once you hang up, run back to the house, sleep for another 24 hours and then return to the same phone box for another message.
There are a combined 12 messages from ‘Our Prudent Friend’ and ‘Mr. Kite’ meaning you should get this all within 12 in-game days. As long as you are at a good enough distance from the phone box, it should ring after every sleep.
If you find that it does not ring each time, try finding another phone box which is slightly further away and you should get it.
Video below is of me doing 2 runs to the phone box to show the rough distance and area that I did it in.