Just to let you know, there is a way for this to be entirely in-game.
Arma 3 has the ability for developers to implement a person to person side channel voice comms across different factions.
Think of it like a private global channel between two people.
Last server I played on implemented it as another option within the messaging system, where you could choose to call someone instead of texting - Plus to receiver could choose whether to answer or not.
Pretty sure I have the files somewhere.