Specify the matching behavior of this dial plan in the presence or absence of a matched external line code. A value of 'Prohibited' indicates that this dial plan can NOT be matched if an external line code has previously been matched in the current dialing string. A value of 'Required' indicates that this dial plan can ONLY be matched if an external line code has previously been matched in the current dialing string. A value of 'Optional' indicated that this dial plan can be matched whether or not an external line code was matched previously in the current dialing string. The default value for legacy dial plans is 'Optional'.