When will we see mobile and home telephone numbers (E.164 numbers) replaced by addresses, such as a SIP address? The UK Payments Council is proposing to use a mobile number as a unique identifier in a mobile payment scenario. A proxy database will hold a mobile number and the sort code and account number to allow payments to be made. We want to understand how far- or short-sighted this approach is. Is the number a ubiquitous resource that will always be around, or will it be replaced by something else?