If you think someone could be spying on your calls, it's time to download Signal Private Messenger for iPhone. With this app you'll be able to call in a more secure manner.
Encrypted communications to increase your privacy.
Even if you have nothing to hide, you would surely prefer not being spied on by governments or other entities. With an application like Signal you can encrypt the communications carried out from your phone.
How to use it on your iPhone?
Using this application is really easy and it works just like any usual dialer on your phone. In fact, to make the calls, there's no need to use any sort of special identifier, as your usual number is used.
The difference is that the communications are carried using your data connection (Wi-Fi or 3G) and they remain encrypted end-to-end.