Microsoft Skype for Business: How to Interconnect to PSTN and PABX

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Microsoft Skype for Business is a widespread solution for telephony, video conferencing, instant messaging and many other communication services. Depending on the application, different interconnection variants with public telephone networks and private telephone systems are available. The following article describes these options and gives tips on what you may consider when you are planning and selecting solutions.

The access to traditional Public Switched Telephone Networks (PSTN) will still be necessary until providers support pure SIP connections and run their own media gateways to the older telephone networks of other providers. Later each provider should offer only Voice over IP services.

Enterprise customers have to decide whether they want to run parallel systems of traditional private telephone systems and an e.g. Microsoft Skype for Business solution. This article assumes that an interworking is required and describes the various interworking variants.



A PSTN gateway is a device that interconnects the Microsoft solution with the Public Switched Telephone Network (PSTN) or a Private Automatic Branch Exchange (PABX). Certified third party solution partners develop these products, because Microsoft does not offer these components itself.

The interfaces to the PSTN or a PABX are carried out analog or digital. A digital ISDN interface is offered as Euro-ISDN basic rate interface with max. 2 simultaneous connections or Euro-ISDN primary rate interface with max. 30 concurrent connections. The actual available number of max. concurrent connections depends both on the technical ability of the interconnected systems and of the licencing of the gateway or the service offering of the provider.

The PBX interconnection can be realized using analog interfaces (eg with inline Dual Tone Multi Frequency, DTMF) or Time Division Multiplexing (TDM) T1 (1.544 Mbps) or E1 (2.048 Mbps) interfaces and ECMA QSIG (Q Interface Signalling) protocol (source: Ask your provider for supported features.

Tip: when you are planning and selecting solutions check for available functions, the licensing of the systems and the support of the provider.

Microsoft has explicitly stated that PSTN gateways and other elements are tested and certified by selected partners (listed in the Unified Communications Open Interoperability Program – Skype for Business Server). See the source of the cited document at the end of the blog.


SIP Trunk

SIP trunks may be used to interconnect the Microsoft solution with public providers and hybrid and pure VoIP enterprise solutions as an alternative to PSTN gateway.

SIP (Session Initiation Protocol, RFC 3261) allows the direct transfer of signaling information between SIP clients, SIP server and SIP-enabled gateways without protocol translation, thereby increasing functionality.

A SIP trunk provider supports the dial-in extensions for incoming calls from the public network or calls from the SIP-enabled solution to subscribers in the public network dialing a ITU-T E.164 number.

A SIP trunk between internal VoIP solutions allows calls between both SIP domains. What features are available in both worlds depends upon the functions implemented. See RFC 5359 (Session Initiation Protocol Service Examples) for frequently used supplementary services.

Ask your preferred public provider for SIP trunking. When linked to private VoIP solutions, these must support SIP trunking capability as well.

Tip: for all the mentioned coupling variants don´t forget the support of the respective national and international emergency call versions. RFC 6881 (Best Current Practice for Communications Services in Support of Emergency Calling) describes different solutions.


Some portions of this blog are based on information that has been published at the following website: „Komponenten für die PSTN‐Konnektivität in Skype for Business Server 2015“

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