VOIP (Voice/Video Over Internet Protocol) based managed and installed business phone systems (a VOIP phone system) have high levels of security. There are many levels of security they employ to help your conversations be secure. So much so, the United States government is in the process of increasing the powers of the 911 Act, so they will have more control over the concealment of information possible on VOIP.
Whether you are using managed or non-managed systems, you are still going to have a huge amount of security. Standard applications, even those free from the internet, come with encryption and other technologies that will protect your communications. There is one area you need to be careful with, and that is the use of your smartphone.
Understand VOIP. VOIP phone systems are just converting sound signals into digital packets. Managed systems have their application on their owner high-speed dedicated server hosting. Installed systems mean you will probably have some combination of dedicated servers, or other devices to handle you VOIP and standard calls in your office. Both systems can be set to automatically choose the best price option between a local call and a VOIP call. Of course, there is a huge range of functions, and features that can be manipulated at a click of a mouse.
Mobile Phones and Traditional Phones. It is important to remember when you are calling a standard phone, or a mobile phone, your call is now not protected with the encryption and other technologies supplied to you by the telephone system. This means your protected call is not protected at the other end. This is important to remember if you are talking important and private issues. There are many countries that record all incoming and outgoing international calls. The best systems will let you know the level of security on each call.
Smartphone. With your smartphone, you will still have the features of VOIP security if you are connecting through your application and not your normal phone number, but there are some serious issues to consider. You need to know whether your VOIP system is also able to hide your IP address, and therefore, location (to some extent), but even more risky is the fact your phone can be geo-located very easily by a network provider. The only way to avoid location detection is to use IP cloaking software when you are using a free wifi hotspot, such as those provided by many airports and cafes. This will also be cheaper because you are not using your data connection. I prefer to put my phone on airplane mode, to make it invisible to the mobile network provider, and then allow access through wireless connection.
Do not think you are invisible just because you turn of your geo-location functions on the internet. The network provider can still locate your phone. If you want to be completely safe, you should take out your SIM card and use your phone that way.