3CX

Website – https://www.3cx.com

3CX is an open standards PBX that works with popular SIP trunk providers and IP phones to allow you to setup a fully functional IP PBX with a complete set of unified communications features. 3CX is well known for its easy installation and management; setup is concluded within minutes and it’s available on premise or in the cloud.

Examples of connecting a 3CX PBX to the SkyTel SIP-Trunk service can be viewed here