Don't trust it if you don't do it yourself.
That's what people should take as a default. Any telecoms provider (old or new) will help law enforcement listen to your calls. If they didn't, the various governments would flatten them. Email and modern telecomms have been the best thing to happen to Intelligence agencies in centuries.
The technology for secure communication is there, however. In fact it's used - for example Lync (way better than Skype) supports encrypted voice (and IMs) and you can use that, but you need your own network, otherwise you still have a third party in the loop.
But there are fine Open Source products for this as well. A few things are needed though - wrapping this up in a more friendly fashion for the WIndows and Mac users is one. (Linux users can handle themselves. And so can plenty of Windows and Mac users but even these have to admit they're a small part of the userbase). Secondly, the ability to move your account around without faffing around with certificates, etc. A nice touch is that in recent years we have been provided with an additional component to make doing this ourself easier - the unique identifiers in modern computers and smartphones used for DRM, can also be used for giving ourselves unique profiles without faffing around with security certificates or trying to work out how to call someone from a different device. You could use the APIs in Windows 8 to add devices to your account, just as you can with any Metro program. And I expect you could put something together in OSX also. Then you have an account with approved devices that you can use to make encrypted calls.
What's lacking? Well critical mass and a provider that can plug your VOIP service into the normal phone networks. Companies are available that provide the latter. Obviously once you dial outside the network then you're no longer encrypted, but the idea is to get more and more people on the network. VOIP is where we'll end up sooner or later anyway. Applications should have a little padlock indicator like HTTPS in browsers - indicating that this call is secure or not.
Anyway, just thinking online. If you're using a public network, you are solely reliant on your country's judiciary to protect you from snooping so the question is do you trust them? Sometimes you're even dependent on another country's judiciary! If your call goes through the USA and you're not a citizen, take it for granted that they'll listen in if they want to. Exactly how far did we get with prosecuting the Bush administration for illegal wiretaps. Not far - the Obama administration killed the investigation as soon as they got into power. If you want privacy - you need to do it yourself.