Should you choose a voip number for personal use, the number has two channels.

There are several ways to utilise those channels, my Asstra in addition to the wired set came with a wireless set if my wife was using the wired set I could also make a call on the wireless at the same time. You must remember while both channels are in use your paying $0.001 per minute x 2

Both of my phones are hold and call transfer capable using the zoiper soft-phone I call set up a conference call with video.

Should you choose a 1-800 number the number of channels is unlimited, so you could build a PBX system using an old PC and Asterisk an open source PBX. Using your own PBX you can now hook up ten or a thousand telephone one or more of those telephones could be reside in remote locations half way around the world.

Often a company will have two or more locations a PBX such as the Asterisk ties in every location into one big presence.

Asterisk.org

https://www.freepbx.org/

https://freeswitch.org/



This URl give a full list of available open source telecommunications software.

Open Source VOIP Software - voip-info.org