I don't think that Miranda NG really needs this, and if, it should be a completely optional feature as plugins. Why? Because there is already an open source project for this, called Jitsi. The focus at Miranda NG should stay at it's existing (text) chat features, because it's the biggest strength of Miranda. Seriously, I haven't seen any other program which excels the features, plugins and options of Miranda NG. (Yes, Pidgin isn't really an alternative. It isn't even able to message specific Jabber clients, which important for serious use of Jabber. And it lacks so many other bloody basic options)
So if you want to make secure calls, use Jitsi. Jitsis strength is voice and video chat, Miranda NGs strength is text chat. For unencrypted voice chat there are also other applications like Ekiga.
TL;DR: Yes it would be cool to see it in the future, but Miranda NG doesn't really need this, because programs like Jitsi are already doing this. And Miranda NGs strength is text chat.