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APPLIES TO: 
2015
2019
Subscription Edition
Plan for PSTN connectivity in Enterprise Voice in Skype for Business Server.
An enterprise-grade VoIP solution must provide for calls to and from the public switched telephone network (PSTN) without any decline in Quality of Service (QoS). Users who place and receive calls should not be aware of the underlying technology: from the user's perspective, a call between the Enterprise Voice infrastructure and the PSTN should seem like just another phone call.
Skype for Business Server provides reliable, scalable PSTN connectivity by using the following options:
SIP trunks to an Internet telephony service provider (ITSP)
Direct SIP connections to a PSTN gateway
Direct SIP connections to a PBX
Depending on its size, geographic coverage, and existing voice infrastructure, an enterprise may use one, two, or even all three of these options at various locations. For details about these options, see the following sections.