SIP trunks are used to integrate NocTel services with existing phone systems and multi-call devices. By using a SIP trunk, you can gain additional features not available with a Generic Device, such as specifying your own outbound Caller ID.
One of the popular open source PBX solutions for companies to install on-premise is FreePBX. When using FreePBX with NocTel, we have created a document to guide our customers through setting up the VoIP trunks to NocTel.