1 Introduction

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The Session Initiation Protocol (SIP) for public switched telephone network (PSTN) Calls Extensions protocol consists of proprietary extensions applicable for interfacing a protocol client with other traditional telephony networks, such as the public switched telephone network (PSTN) and an enterprise private branch exchange (PBX) or IP-PBX.

Sections 1.5, 1.8, 1.9, 2, and 3 of this specification are normative. All other sections and examples in this specification are informative.

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