Analog Devices in Skype for Business

Analog devices are one of the least understood aspects of a Skype for Business deployment. In this post, I’ll provide a 30,000 foot overview of how analogs work with SfB on-prem deployments.

An analog device is anything like a phone, fax, alarm system, door buzzer or overhead paging system that uses an analog phone line. That phone line is a pair of wires that form a circuit that connects the device to your PBX or the telco.

With SfB, there is no Sfb hardware device that the analog phone can plug into. You have to use a gateway for this, from Microsoft partners such as AudioCodes and Sonus. However, plugging an analog device into a gateway doesn’t magically make it an SfB device. It doesn’t have presence, it doesn’t logon. It’s still just a boring analog device. The gateway communicates with SfB over a SIP trunk, just as if it were a SIP trunk from your Telco.

Gateways PBX and PSTN

What we’ve got then, is an analog device that has an analog circuit to a gateway, that gateway has a SIP Trunk to SfB. SfB will have a connection to the PSTN, which will use an SBC or another gateway.

It is possible to have analog ports on the same gateway that’s connecting your SfB environment to the PSTN, but for clarity I’m showing them as two separate devices.

When you place a call from the analog phone, the analog gateway uses its routing tables to determine where the call should be routed next. This might be a simple “send everything to SfB”, or it could be more complex and allow connections direction to the PSTN gateway, or to SfB.

When a call comes in from the PSTN, the PSTN gateway has similar options. It can send all calls to SfB, or it can route the calls to different locations. When calling from SfB to the analog devices, SfB sends the call directly to the gateway.


In all cases, you have to configure the gateways’ routing tables. Every gateway you add to your environment adds to the administrative overhead of your solution, and the complexity of your call routing options.

Call flows and routing decisions are a significant part of dealing with analog devices. In the next posts, I’ll cover how gateways can make routing decisions, and how SfB can be configured to simplify routing to analog devices with some loss of more advanced capabilities.

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