Voice Policies Redux

After covering the voice policies, PSTN usages, and routes separately, it’s now possible to understand how they all work together. The starting point is the E.164-formatted number that has been normalized by a user’s dial plan. The next step is to review the user’s voice policy to see whether the user is allowed to call the number.

The first PSTN usage in the voice policy is selected, and any routes associated with that PSTN usage are reviewed. The normalized number is compared to the match patterns on each returned route. If there is a match, the route is selected and the call is placed through one of the trunks.

If there is no match pattern for any of the associated routes, the process repeats. The next PSTN usage in the voice policy is selected, and any routes associated with that usage are reviewed for a possible match. Again, if there is a match, the route is selected and the call is placed through one of the trunks.

This process continues as each PSTN usage and associated routes in the voice policy are reviewed. If the end of the list is reached and no match has been made, it is determined that the user is unable to place that call. Figure 17.17 depicts a simplified version of the call routing logic.

Figure 17.17. Call routing process.

For example, assume that a user’s voice policy contains only two PSTN usages, such as US-CA-SFO-Local and US-CA-SFO-National. The Local PSTN usage is associated with a route that has a pattern matching a +1 followed by area codes local to San Francisco such as 415, 510, or 650. The National PSTN usage is associated with a route with a pattern that matches a +1 followed by 10 digits.

If the user dials a number matching either of those patterns, the call will route successfully. But if the user tries to dial a hotel in England that starts with the country code +44, the call will fail. In that example the +44 is first compared against the US-CA-SFO-Local PSTN usage and any associated routes. There is no match, so the next PSTN usage, US-CA-SFO-National, is evaluated. Again, there is no match. Since there are no more PSTN usages to evaluate, the call fails and the user is informed that they are not allowed to dial that particular number.