Digium also offers a range of IP phones with Asterisk-specific features such as the capability to be supported and configured remotely, and PSTN interface cards that are sold with support to work in an Asterisk environment.
So while you are not strictly paying for the software, you are paying for a solution built on it. "Many companies don't just want Asterisk, they want to buy a complete telephony solution which includes software, support, phones and failover capabilities," says David Duffett, Asterisk's community director.
6. Low-Cost Platforms for Proprietary Products
Digium is unusual in that in addition to distributing Asterisk under the open source GPLv2 license, it also makes the software available with a low-cost commercial license. This provides one final reason to pay for open source software: If you pay for a commercial license, you can modify the software without the obligation of providing the resulting code to the original development community under the GPLv2 license. This can be useful if you want to incorporate the modified code into your own commercial products.
That's a (Commercial) Wrap
What you are generally paying for with a subscription to an open source-based product is a commercial wrap to put around the open source code. That wrap includes support, testing, hardware certification and predictable product lifecycles.
"By paying a subscription you get the same experience as you do with proprietary software, only for far less money," concludes Haff."
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