This is because the transmission of voice communication is through IP packets through the Internet. Normal PSTN did not convert voice into VoIP packet.
To transmit voice through VoIP, we have to digitize it with an ADC (analog to digital converter), transmit it, and at the end transform it again in analog format with DAC (digital to analog converter) to use it.
Once the voice is in digital form, it will be split into the IP Packet for transmission.
Advantages of VoIP is that, we can talk long distance with very low charge rate compare to normal PSTN.
In order to use VoIP we need to have VoIP phone or a complete PC with sound card, microphone, speaker and VoIP sotware. The parties that we called also need to have this.
Most of the software for VoIP call are free and dan be downloaded from the internet. For example Skype, SJ Phone, Yahoo Messenger with voice and so on.
Due to existing normal PSTN telephone in most of our offices and house right now, it is not easy to replace all of them with VoIP phone. It will take sametime for that. Therefore a device called ATA is used to enable normal PSTN work as VoIP phone.