That is a somewhat valid point but I think you and me are using decentralized in different senses here. When I said decentralized I meant a decentralized application, something without central servers. You are more talking about standardized protocols that can be implemented by different vendors and which together form a federated network. There is certainly also some overlap there.
But your examples of phone, mail, and email are essentially all centralized systems with a certain number of vendors for each but together forming a larger federated network due to standardized protocols. Email is probably the closest of your examples to what I call distributed because at least in theory everyone could run his own mail server although in practice it is probably not so easy because spam made everyone pretty paranoid when it comes to forwarding emails from random mail servers.
But your examples of phone, mail, and email are essentially all centralized systems with a certain number of vendors for each but together forming a larger federated network due to standardized protocols. Email is probably the closest of your examples to what I call distributed because at least in theory everyone could run his own mail server although in practice it is probably not so easy because spam made everyone pretty paranoid when it comes to forwarding emails from random mail servers.