The imaginary phone you have with you when you forget your real phone.
Buxton's Phone is created each time reach for your phone despite the fact it's not with you. It vanishes the moment reality returns you to its terrible clutches. It is extremely lightweight, portable, undetectable by any modern technology, and waterproof, but unfortunately not compatible with any carrier or battery charger.
Bobby repeatedly reached for his Buxton's Phone every minute for two hours after leaving his real phone in his "sexy pants".