Systems, methods, and machine-readable media are disclosed for providing customization to routing of messages in a system where routing is based on stored addresses, such as, for example, a Session Initiation Protocol (SIP) system, to pass through a firewall. In one embodiment, a method of providing customized routing of messages to pass through a firewall can comprise receiving a message from an initiating agent to a target communication identifier at a system for signaling between endpoints. The system can have one or more stored network addresses for each of a plurality of communication identifiers. One or more network addresses for the target endpoint can be determined from the stored network addresses associated to the communication identifier. One or more policies can be applied to the one or more network addresses for the target communication identifier to route the message through the firewall.