While speaking at Facebook's F8 developers' conference, Acton said that WhatsApp team "refined and iterated voice calling" last year before launching it on Android. Unfortunately, Acton did not reveal (via Venture Beat) an exact time-frame for the release of the voice calling feature on iOS, and when pressed later disambiguated to say, "Well, several weeks."
(Also see: How to Activate WhatsApp Calling)
Acton's statement however is the first official confirmation of the start of the rollout of the voice calling feature, which began with Android users earlier this month. Unfortunately, WhatsApp voice calling feature is being rolled out in phases, which means it is currently available to limited users and invites only work within specific time periods.
Leaked screenshots claimed to be from a jailbroken iPhone running the WhatsApp v184.108.40.2064 beta app showing off the UI of the WhatsApp voice calling feature surfaced earlier this month. The screenshots showed a UI that was in-line with the interface spotted last month.
The leaked WhatsApp for iPhone screenshots showed mute, message, and speaker options while in-call, with 'WhatsApp Call' clearly marked on the top left of the calling screen. Notably, the current WhatsApp version available to download from the App Store is 2.11.16, tipping that the voice calling feature might be rolled out with the next app update.