This Recommendation defines the procedures applicable for the purpose of numbering plan interworking between networks which use the E.164 numbering plan and networks which use the X.121 numbering plan. The principles for interworking between the E.164 and X.121 numbering plans covered in this Recommendation are illustrated by various examples of packet mode calls. Although both the escape code method and the Numbering Plan Identifier (NPI) method are described, the recommended solution is use of the NPI method.