VoIP uses UDP because it’s faster than TCP.
Transmission Control Protocol (TCP) is a connection-oriented internet protocol that is established and maintained until the exchange of messages between programs at each end of the line is finished. When TCP is used, the data is broken into packets, which are sent to the receiver’s program in order and uncorrupted. This ensures that the message is delivered accurately but at the cost of speed.
User Data Protocol (UDP) breaks the data into packets and sends it to the receiver like it should be, but not in order. Sometimes, packets even arrive corrupted or don’t arrive at all. However, programs using UDP keep the stream of information going uninterrupted and without any delay whatsoever, making UDP perfect for live communication, such as phone calls.