This might happen for several reasons. To fix it, make sure that you have your account verified. If you receive a message that states: "Phone Number Not Verified", visit the following page in order to solve this. You should use a stable Internet connection while calling with Tango. It's advised that you use a WiFi connection for this operation. Also, make sure that you don't have any background data restrictions enabled on your device. For more information, visit the official FAQ website.