FreedomPop's basic SIM-only service offers 200 minutes, 200 texts and 200MB of data each month absolutely free. While that's technically true, FreedomPop effectively relies on data alone, with calling and messaging handled through a standalone app, not your smartphone's native dialer. And now, where you get that data from is by the by, as FreedomPop has opened up its iOS and Android apps to everyone, regardless of what's scrawled on their SIM. A couple hundred minutes and texts might seem redundant in the FaceTime/Hangouts/WhatsApp era, but having a virtual number means you'll have no trouble calling your nan's landline, because she doesn't have Viber, does she?
WiFi-only devices can support the service too, of course, and anyone that signs up to FreedomPop also gets 300 free minutes of international calls in the first month -- handy if your Skype credit is running low. Make sure to stay within those free allowances, though. Overage charges are how FreedomPop makes money, after all, as well as selling added-value packages like unlimited calls and texts for £4 per month, or a second, foreign virtual number for cheap international calls.