FreePBX

FreePBX is a web-based open-source graphical user interface (GUI) that manages Asterisk, a voice over IP and telephony server.
FreePBX is licensed under the GNU General Public License version 3. It is a component of the FreePBX Distro, which is an independently maintained Linux system derived from the source code of the CentOS distribution, having Asterisk pre-installed. It is also included in various third-party distributions such as The FreePBX Distro and AsteriskNow.
FreePBX was acquired by Schmooze.com in early 2013. That firm was, in turn, taken over by Sangoma Technologies Corporation on Jan 2, 2015.
FreePBX is a community of developers and contributors who devote their work to making complicated phone system software easy to use and functional.

An example of connecting FreePBX to a SIP trunk from SkyTel:
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6. Connecting a SIP trunk by username and password

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10. SIP trunk connection without registration (via IP)

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13. Setting up routing

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