Not sure about the VOIP feature. The amount of idiots that often run around in these type of games that think only their strategy is valid and everyone need to follow their great tactical genius is quite high. I'd rather not have them shout into the game all the time.
If it is added, there should be a "do not accept VOIP" option to just mute everyone as standard. However, this should also be indicated to the team mates that this player does not hear VOIP.
Overall, I think a kind of ping/flag system would be best. Mute functions for text chat and pings should be implemented too. As well as a report button, but that is another topic.
I completely agree, I'd like for CoH3 to adopt a Men of War style of ping/ chat system where:
1.) VOIP is available for use if wanted (and you can "deafen" yourself like you said)
2.) The ping system is more fleshed out; in Men of War, there is a "ping" button. If you simply press the button a generic ping happens at your cursor. If you hold it, a radial menu opens up with different, more detailed pings.
3.) (This one could be useful for play between users who speak different languages, in addition to the expanded pings menu) Have a feature like Men of War where you can use chat normally, but if you HOLD the chat button it brings up a radial menu of preset phrases like "Thanks!" "Sorry!" "OK!" and these phrases could perhaps actually be localization strings. So if you use the "Yes!" phrase, English speakers see "Yes!", and German players see "Ja!" and so on.