Anyone that wants to ignore your orders in text chat probably won't take your orders in voice chat either. Voice chat in gaming has some inherent problems, primarily with trolls and the fact that it's difficult to impossible to moderate them. Riot has said before that while they are interested in implementing voice chat, they want to do it in a way that avoids its typical problems in other games.
Until that time comes, I recommend using Discord or Curse Voice; when CV was newly in open beta lots of people were trying it out, and it was common to have multiple players in your lobby open to joining a channel for the match. The popularity wore off, but the Discord App is popular now, and the best part is anyone can join the chat through the website without downloading the app.
Basically, just try being proactive and asking your team to join your 3rd party voice chat. The fact that people have to do something to join voice chat instead of having it by default weeds out a lot of the trolls.
Edit: I'd also like to mention that you're not going to have coordination like pros do whether your entire team is on voice or not. Your team might even perform worse than without it.