I have an iPhone 5 that I have owned since late 2012. Shipped with iOS 6.??.?? and I upgraded to 6.1.2 before I jailbroke then jumped off of the upgrade bandwagon.
I have several online meetings a week that I call into with my iPhone 5, and use the mute button (successfully) for almost all of them. I am mostly doing a lot of listening.
Unless you have some desire to upgrade, but I am assuming you are not, or you wouldn't be here, I would probably do anything else I could before I considered an iOS upgrade as a possible fix action.