The PhoneBoy Blog


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Dial Plans and You

A dial plan essentially defines what numbers are “valid” when trying to make a telephone call. In the PSTN world, something similar to a dial plan is in place at the central office. In the VoIP world, the dial plan is actually in the VoIP adapter itself, at least when the SIP protocol is used. A few VoIP adapters, most notably Grandstream, don’t provide a dial plan at all. PBXes of any sort also have a dial plan to determine how to route calls.

A dial plan provides a context for interpreting the numbers dialed on a telephone keypad. For example if I dial the number 555 1234, what will determine where the call goes? The dial plan in the central office. When dialed from my house, 555 1234 will route to +1 360 555 1234. If I dialed that number from the San Francisco Bay Area, it might go to +1 408 555 1234. If I dial 555 1234 from Atlanta, I will get an error as 10 digit dialing is required in that area.

Now why did I present a failure condition? To illustrate the point that every place has different dialing rules. For example, I can dial a number as 555 1234 or 360 555 1234, assuming the number is a local call to me. If the number is not local to me, then I must dial a 1, even if the number is within my area code. Contrast that to the San Franciscio Bay Area where a 7 digit dialed call can be long distance and you must dial 11 digits even for a local call. (How are you supposed to know a call is long distance? I bet PacBell SBC AT&T makes some coin off those mistakes.)

So why am I talking about dial plans? Because it is somewhat of a hinderance to getting people to use VoIP. People don’t like to change their dialing patterns, especially when it comes to the telephone. The problem is, VoIP companies implement dial plans that are sometimes incompatible with people’s local dialing customs. While most devices can be programmed to conform to just about any dialing pattern you want, getting the providers to do the heavy lifting is another matter.

On the Voxilla Forums today, I helped someone craft a dial plan to cope with their particular dillema. They lived in an area where they currently support 7 digit dialing but will eventually migrate to 10 digit dialing later in the year. They wanted both methods to work. I was able to do it, but it required some thought. Now multiply this problem by several thousand communities all with different dialing requirements and now you know why most VoIP providers only give you the option of 7 or 11 digit dialing, or maybe 10 and 11 digit dialing, but NOT 7, 10, and 11 digit dialing. The providers certainly don’t have the manpower or necessarily the ability to make a custom dial plan for an indivudual device, so a set of generic rules is put in place.


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