This is quite a new concept for TZX readers, but still I feel it a worth to know about. If you are a corporate, you will find it more 
useful to adopt Asterisk PBX Systems.
Asterisk is the software implementation of the PBX (Public branch exchange) telephone where you can find telephones which connect with each other and it will also be connected with other networks like Public Switched Telephone Network - PSTN and the most revolutionary internet based telephone service VoIP - Voice Over Internet Protocol.
The speciality of this software is that it is open source which has lead to contributions of different developers in terms of advanced features. The advantages with this system is that you will find it helpful in ease of usage to connect your business with customers, clients and employees. Striking advantage here is the low cost features as the system is license free and software is self administrative.
If you are a company who rely more on telephone activities (which company doesn’t need it ?), it is better you go with asterisk systems and also it is recommended to take advice from experts in VOIP consulting and IP phone systems so that you will pick right options which saves you a lot of money with internet based phone calls.
I know a lot of my friends who use this in their office. Although I have no personal experience with it yeah.
Opensource software is always great for the consumers as well as for the developers. Because the consumers gets something free and high quality while the developers get to hone their skills to create better software.