The app will shortly be avialble for iOS and Windows phone, Nanu said
Signal is compatible with RedPhone, its Android sibling, and scrambles voice calls placed via the iPhone.
The popular messaging service is taking on the likes of Skype and Viber with its new VoIP service.
Sprint plans to follow T-Mobile's lead and roll out Wi-Fi calling options, but just on two phones to begin with.
A mere week after launching a new direct calling service, Viber is expanding yet again.
Viber's new low-cost phone calling option is a steal if you're dialing phones in the U.S., but do you homework if you're making international calls.