ClearOS should be able to provide VOIP services to the network.
If considering using ClearOS as a single solution for all the services, we need to include voip, with support for ATA hardware, such as internal PCI & PCI Express telephony cards with FXO and FXS ports, such as the Tdm800p rom Digium and also external ATA units such as the Linksys Spa3000, or the Spa3102, Grandstream Gxw4108 8 Fxo ports, etc. This type of Hardware is most common on areas where the infrastructure is either old or too expensive compared to digital lines such as an E1/T1.
The server may be able to help remote agents, or even entire offices, connecting two locations permanently and making the extensions available all the time.
It needs to provide the basic functions of voip like authentication, routing, call waiting, transfer, recording, voicemail, conference. This is something that ClearOS could do and help reduce the numbers of servers on the network and so on.
It may require a STUN server, there is one in OpenFire but it might be better to have a different separate app.