1. A method comprising: at a hardware call router system that is communicatively coupled to at least an external first application server via an application layer protocol and that is communicatively coupled to a telephony network: responsive to the call router system initiating a telephony voice session for a telephony endpoint associated with an initial Uniform Resource Identifier (URI) of a first call router account for the first application server, the call router system: determining a first secret key of the first call router account, generating a first request to a first application resource of the first application server, wherein the first request includes state information of the telephony voice session, and wherein the first application resource is specified by the initial URI, signing request parameters of the first request by using the first secret key, and sending the signed first request to the first application server, wherein the signed first request is an application layer request; responsive to an application layer protocol first response that is provided by the first application server and that includes a plurality of telephony instructions, the call router system sequentially processing at least a subset of the plurality of telephony instructions and executing at least one telephony action according to the sequential processing of the telephony instructions, wherein the first application server provides the first response responsive to verification of the signed first request by using the first secret key.