scope:

Introduction

Recommendation I.330 describes ISDN numbering and addressing principles, while Recommendation E.164 describes the numbering plan for the ISDN era. Recommendation E.164 also identifies the need for interworking arrangements between ISDN and present dedicated networks.

This Recommendation sets a specific time (Time T), after which all ISDNs and PSTNs can use the full capability of Recommendation E.164, "Numbering plan for the ISDN era", and identifies the numbering requirements on ISDNs and on dedicated networks intending to interwork with ISDNs, before and after Time T.

Among the significant principles which form the basis for this Recommendation, the following are considered especially useful for ready reference:

- An E.163/E.164 telephony subscriber may become an ISDN subscriber without a number change.

- Numbers according to Recommendation E.164 apply to both PSTN and ISDN subscribers in the ISDN era. A mixture of PSTN and ISDN terminations on the same exchange is allowed.

- E.164 numbering arrangements may be used to distinguish between ISDN and PSTN subscribers. This is not necessary but is allowed, provided that possible effects on routing and digit analysis remain within the limits of Recommendation E.164.