The use of ENUM and SRV-Records might be the ideal solution, but still has some obstacles that need to be overcome. Most available VoIP products don't support both features. There are some products, especially in the SIP world, that support SRV-Recordss but just a few that support ENUM as well. Products that are already in use probably won't be able to resolve telephone numbers via ENUM.
Some vendors will offer ENUM/SRV-Records capabilities by software upgrades, but not everyone will. To protect investments that have already been made it is necessary to find a solution that integrates older IP telephony equipment.
To enable a ENUM unaware H.323 gatekeeper or SIP proxy to use this feature, one can make use of the ability to configure a default server (further on called Call Routing Assistant (CRA)) that all unknown calls shall be routed to. Such a configuration option is provided by nearly all IP telephony servers. The Call Routing Assistant will act as the VoIP equivalent of a default gateway and perform call routing instead of the ENUM unware server.
Such a Call Routing Assistant could also be used to use a firewall to protect the internal IP telephony server and open just a single hole to the CRA as a gateway for external communication.
If the internal IP telephony server only supports one signaling protocol, the CRA could include SIP-H.323 gateway functionality.
The Call Routing assistant isn't a special product. In theory every IP telephony server that can be configured to route or resolve calls without registering the other server can be used in this place. Efforts are made by the Center for Computing Technology in Bremen, Germany to provide an easy to use, open source CRA implementation.