When using the Blitzz Video Calling feature, you should always strive to send media over UDP instead of TCP to handle packet loss with real-time communication.
With TCP/IP, all dropped packets are automatically retransmitted while in UDP, there is no automatic transmission of lost packages. During a live video call, a dropped package is not important as that may be delayed and could introduce lip-sync issues. On the other hand, during a file transfer, every package is important and so TCP may be preferred.
This is why UDP is used in real-time streaming applications which can deal with some amount of packet loss (or out-of-order reception).