Voip Blog
What's Your Dial Plan?
Thursday, May 22, 2008
When you dial a telephone number, you probably don't think about it too much. You know that as long as you dial a valid number, the call will go through and connect you to your destination. However, the network has to take what you dialed and do something with it. What it does depends on what you dialed.
If you use an analog telephone adapter or an IP telephone, it is important that they emulate--as much as possible--the traditional telephony experience. Unlike in a traditional telephony environment, where the endpoints don't actually know anything about how to dial telephone numbers, VoIP-enabled endpoints do. In fact, they need to know in order to do their job of emulating the traditional telephony experience.
Dial plans for VoIP devices are simply rules that tell the VoIP device when it should considered the dialed number complete and ready to attempt to connect. It also does some gross-level filtering to ensure that the numbers dialed are a valid phone number.
For example, if you dialed 031795443 on a North American dialing plan, that would be considered an invalid number. The VoIP device would likely issue a reorder tone when it saw the first three being dialed as the only digit valid after dialing a zero is a one (as in 011 for an international call).
The other reason for needing to know what's valid is so that the call can be connected as quickly as possible. For example, when I dial 911, or any other X11 number for that matter, it needs to know to send those digits NOW! If I dial 1 as the first digit, the VoIP device should expect 10 more digits to be dialed before attempting to connect the call.
Where it gets tricky is supporting 7 versus 10 digit dialing, which we will get into next time.
