Your device is connected to a weak or inconsistent network connection. Try connecting your device to another network connection to see if that fixes the issue. If you are connected to your mobile carrier's network, trying connecting your device to an available Wi-Fi connection and see if that helps.

Force close any other apps you aren't using that are running in the background. Make sure that Tango is running in the background and hasn't been force closed.

Make sure your friend hasn't accidentally been added to your BLOCK list.

Your device might have the wrong settings set up to be able to receive calls and notifications. Check the links below for further instructions: