You can use analog phones for VoIP communication, you just need a device that can convert the voice signals.
Open this page: http://www.ozekiphone.com/analog-telephone-on-a-voip-network-479.html
Analog phones are used in the Public Switched Telephone Network (PSTN), the old system.
VoIP (Voice over Internet Protocol) is used to provide transmissions of voice and multimedia sessions over the Internet. Since data transmission is digital, analog voice signals must be digitalized.
The key is the ATA (analog telephone adapter) that converts the analog data to voip and the way back.