In North America, permissive dialing is the ability to make phone calls in an area subject to a newly introduced area code by using both the new and preexisting dialing methods.
To alleviate misdialing frustration, the local routing can be set up such that both the old and new area codes will work for the same telephone exchange.
At the end of the permissive dialing period, the old area code is no longer valid for numbers in the affected area.
Again, the affected area must not introduce any new ambiguous telephone exchanges.
At the end of the period, 10-digit dialing becomes mandatory.