I feel this is a VERY bad idea. If you think people can be creepy in chat...imagine them in a voice call. Just HELL NO! I used to play L4D2, and if you didn't have a microphone or didn't want to use the voice chat, basically your team would just f*** you over -- it really sucked, so basically if you include a voice system in a game like this, folks will expect you to use it and they will be miserable a*-holes if you don't.
If you find someone you like to play with, add them on skype...but there is no way you wanna be on a voice call with some of the creeps you will meet on league. As there is usually one in every game, that will just ruin your day and make you have nightmares because they are sick f****! Oh that and do you really want some a*-hole screaming you f****** noob at you?
I'm sorry but there are too many toxic players on league for this to work. I could see this working on some games that don't have the uber toxic community.
Edit: Oh and muting?? no first of all if you have to mute them then the damage is already done, secondly if you start muting folks in a call it becomes very hard to parse the conversations, and vola crappy environment! Yes I am ready to rage at folks who support this idea lol xD
I will be blunt. The ability to mute/ignore makes the whole first part of your post lack relevance.
As for the "Edit" part, you're not basing yourself on hard evidence. VoIP has been very successful in many games and it is only abused by a minority. Completely ignoring the benefits a tool like this can bring to overall communication and coordination just for the sake of a few bad apples that can easily be dealt with, with the aforementioned function, is hardly justifiable with the lackluster reasons you've given.
"The damage is done": No offense but this isn't a legitimate reason to keep a good tool away. Someone's annoying over VoIP, mute them. Although it might feel like it, no permanent brain damage is done from hearing a few seconds of annoying sounds before you mute them. You will survive, I promise!
"Crappy environment": I don't think not having the feature that facilitates the conversation in the first place will help you better "parse the conversation". Chat is not necessarily a better alternative either and we've all been a witness to this (even you).
At the end of the day, the pros and cons must be weighed when adding a new feature, and you can't just dismiss it because of a few negatives, especially when in the broad perspective it would bring many more positives through superior communication and coordination, faster reaction time and better team synergy.