Skype for Business Server 2015

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Load your existing and deployed topology in Skype for Business Server 2015 Topology Builder.
If not already deployed, deploy a Skype for Business Mediation Server.
Enable TCP connections for this Mediation Server on port 5068.
Add a new PSTN Gateway to the topology with the FQDN (IP address should work too) of your OfficeMaster Gate SBC.
Set transport protocol to TCP and Listening port to IP/PSTN Gateway to 5060 (default value of OfficeMaster Gate SBC) and select your deployed Mediation Server.
In Skype for Business Server Control Panel Voice Routing section configure, that the numbers of your intercom will be routed through the OfficeMaster Gate SBC gateway when dialed.

For OfficeMaster Gate SBC configuration you can refer to its manual.
Set the SIP Trunk Registration setting to the PBX to which your Helios IP intercoms are registered, use addresses and ports relevant to your particular configuration
Configure PCM1 and PCM2 interfaces. Interface PCM1 faces to the Skype for Business server and its From ISDN direction should be configured similarly to following example.

The numbers should be tranformed in such a way, that they correspond to numbers which are assigned to the Skype for Business users. The destination setting should contain the address, port and transport protocol of the Skype for Business Mediation Server.
PCM1 To ISDN direction should not apply any transformation to the numbers and forwards the calls to the virtual ISDN network.
Interface PCM2 is facing in the direction of your PBX. Rules in its From ISDN direction should tranform the numbers so they can be understood by your PBX. In the following example it just strips away the +420 prefix.

The destination should point to your PBX to which the intercoms are connected.
The To ISDN direction should not do any transformations of the numbers, it just forwards the calls to the ISDN network.

Configuration of your intercom should stay the same, only add the numbers of the Skype for Business users which you want to call from the terminal.
Your PBX should be configured in a way that calls to the numbers assigned to Skype for Business users are routed to the OfficeMaster Gate SBC.