SIP trunking is a smart way for your Lync deployment to obtain telephone services from an external provider.

The trunk enables internal users to call the PSTN network, and allows external users to call Lync clients with a direct number.

The deployment of a SIP trunk does not require specific hardware (such as a PBX or a gateway) because it is based on a straight connection to a service provider.

It is also a great solution if you think to deploy Lync combined with an existing VoIP or telephonic solution and you do not want to use the existing voice services and numbers. This solution implies that your Mediation server has two network interfaces, one will be talking with the Front End Servers, and the other one will be connected to Edge of the service provider. You can set up a trunk from the Lync Management Shell or from the Lync Control Panel. The parameters will be dictated by the standard that is used from your provider.