Figure 4.4 Per-number routing
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[IP Telephony Cookbook] / Setting Up Basic Services
This is quite easy to achieve by setting up a database that stores the information for telephone numbers that belong to the PBX or the IP world (see Figure 4.4).The IP Telephony server and the PBX access the same data to decide where to route a call. Calls to external targets are routed to the PBX and out into the PSTN.
There are variations to this scenario. Indeed, it is quite unusual that an IP Telephony server uses the same database as the PBX, unless they are from the same manufacturer.Then there are two possibilities: setting up a second database suitable for the IP Telephony server (and risk inconsistencies) or let the IP Telephony server route calls to unregistered targets to the PBX.The latter is easier to implement but prevents the discarding of the PBX and the use of IP Telephony providers in the future.
4.1.1.1.3. Per-number routing: more than one server
A similar but more complex scheme is the variant where there is more than one IP Telephony server in the IP world, e.g., a server for each signalling protocol used. In this case, there needs to be a database that contains not only the information about which number shall be reached on the PBX and which in the IP world, but also the information about which IP server (and signalling protocol) to use.