The basic procedure is:
- evaluate if any other system numbers use the digit you want to use for an outside line
- adjust the system dial plan (Voice > Dial Plan)
- adjust the phone dial plan (Voice > IP Phone Globals > Default Settings tab)
- adjust the trunk dial plan (Voice > Trunk Groups > select applicable trunk group > expand detailed view > Configure Advanced Templates button)
- add DNIS substitutions so the proper numbers are sent out the trunk to the PSTN (Voice > Trunk Accounts > select trunk > DNIS Substitution tab)
If you do not want to use a leading digit to access an outside line you will be susceptible to a phone dial plan time out. The users can either wait for the timeout or bypass it by pressing the dial key or # when they have finished entering their dial string.
The exact steps that need to be followed are in this guide written just for this purpose:
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I just wanted to update this thread to highlight that this process was built into a new feature in R10.8. For new installs there is a installation wizard that will take care of this automatically and for existing installs there are several steps and considerations. Here is a link to the configuration guide that covers this: Configuring the Voice Trunk Access Code in AOS R10.8 or Later