SIP trunks enable the use of VoIP services that are provided by carriers or service providers. The service provider provides you with a phone number or a number range. Alternatively, the service provider can assign individual SIP addresses—known as SIP URIs (e.g., "tom.jones@swyx.com")—to Swyx users, or even entire ranges (e.g., *@company.de), which can be used like an "email address for a phone." This allows Swyx users worldwide to reach other SIP URIs and be reached themselves. If this service provider also offers gateway services, it will also be possible to reach any phone in the public telephone network via an SIP trunk and the underlying gateway of the provider.

An SIP trunk connects the software-based telephony system to a service provider, e.g. a SIP-ITSP (Internet Telephony Service Provider) or SIP provider for short (e.g. 1&1, outlook.com, GMX, etc.). Call connections are created and terminated via SIP (Session Initiation Protocol) as a communication protocol.

Within Swyx, the connection is configured as a SIP trunk or SIP trunk group.