Earlier quoted context omitted.
> Notably, it is a must for VoIP Wouldn't making STUN work be a better alternative?
Yes, it’s a feature supported by many VOIP clients, and this comments section is filled with UPnP apologists
And TURN is one of those relays.
(I host a STUN and TURN relay myself, because I had to for my personal VoIP server for enough people to be able to connect on it. Downside is more use of bandwidth.)
edit: replaced STUN with TURN where appropriate, I did confuse both as they were provided as a single package.