Let's assume that in the interest of cutting costs the FSA wants to limit a particular group of users to using only certain area codes. You can do this by defining one or more dialing rules.
For example, users at the ACME Software company in Seattle need to correspond with with users in Silicon Valley (i.e. San Francisco and San Jose) as well as within the state of Washington. The area codes for the different sites are as follows:
To limit access to these three area codes we can set up three dialing rules, one for each exchange:
(1) (204) [???] [????]
1 204 $a $b
1 204 $a $b
1 415 [???] [????]
1 415 $a $b
1 415 $a
1 408 [????]
1 408 $a $b
1 408 $a $b
Now we can give the group of users permission to use the three dialing rules. In this case we need to tell them not to use area codes when they enter the number. To allow area codes we could define additional dialing rules as we did earlier in the "redundant area code" example.
Note that if you only use these three dialing rules users will get an invalid fax number error if they dial a number that is not in one of these three area codes.
If you give users permission to use these dialing rules as well as others (for example, the default), then the new dialing rules become an added convenience for users. For example, if the user just enters a number like 1 (415) 555-1212, the number will be correctly dialed to San Francisco. However, if the user enters some other number like 1 (604) 926-8182, the default dialing rule will allow the number to be dialed.