The secure, fixed-rate voice and data calling plan works on Apple iOS and Android smartphones and, eventually, on Windows Mobile systems, the Geneva-based firm said. Callers can reach 79 countries, including China, Russia, most of Europe and the Americas. Large parts of the Middle East and Africa are not covered.
The service marks a sophisticated challenge not just to traditional phone carriers -- who still by and large charge steep roaming fees to international travelers when calling from outside their home market -- but also to newer, voice over Internet services that have sprung up over the past decade.
For while Silent Circle undercuts major competitors' roaming costs in many countries, the service's basic attraction lies in its distinctive security features.
While no one is immune to such snooping, Silent Circle offers customers a secure means of encrypted communications that disposes of all records of calls, texts or emails.
The new calling plan starts at $12.95 a month for 100 minutes of outbound calls and runs up to $39.95 for 1,000 minutes of calling, comparable to premium price plans offered by Internet communications services such as Microsoft's Skype and Rakuten's Viber - but with much higher levels of security.