Dial Permissions determine the ability of a user to make calls based on the rules set for each permission.  The basic set of permissions are listed below and a brief description is given for each.

911 OnlyThis will only allow 911 and 933 calls to go through.
All 1+ DestinationsThis will allow the dialing to all 1+ destinations including the US, Canada, Carribean, and other higher-cost 1+ destinations. Calls beginning with 011 are denied.
Continental USOnly calls to numbers within the Continental United States are allowed. Hawaii and Alaska are denied.
Deny AllThis will cause any call to be denied.
Deny InternationalAll international calls will be denied.
Deny OutboundThis will allow the user not to be able to make outbound calls but will still have the ability to receive incoming calls.
Extension Dialling OnlyThis restricts the user to Ext to Ext calls only, 911 is also allowed.
Force Account CodeAny calls made by the user will require them to input an account code first. See the following article: Account Codes
Permit AllThis will allow the user to place calls to any destination regardless of dial pattern. 
SRS RestrictedSRS restricted is an automatic Dial Plan that limits users to the US and Canada. This plan is applied when our fraud monitoring tools see suspicious activity from an extension. If you have a User that is showing as SRS restricted please contact Support. If you change a Dialplan from SRS restricted after it has been set by the system, it will automatically change back until the Fraud related problem is resolved. See our article SRS Restricted Users
Toll Free OnlyThis will only allow calls to toll free numbers and 911 only.
US and CanadaThis allows calls within the US territories and including Canadian numbers.
US, Canada, and MexicoThis will allow calls within any destination in the United States, Canada, and Mexico.
US, Canada, and PRThis will allow calls within any destination in the United States, Canada, and Puerto Rico.

Support can add custom Dial permissions on a per-domain basis. This requires customization and falls under the Domain Customization and Custom Dial Translations pricing.

Setting Dial Permission for a User

To set the dial permission for a user open their profile in the PBX and go to the Dial Planning section. A drop-down menu beside Dial Permission can be used to select the dial permission for the user.