Your friend's device is connected to a weak or inconsistent network connection. Have your friend try connecting their device to another network connection to see if that fixes the issue. If they are connected to their mobile carrier's network, have them try connecting their device to an available Wi-Fi connection and see if that helps.

Have your friend force close any other apps that they aren't using that are running in the background and have them make sure that Tango is running in the background.

Make sure your friend hasn't accidentally added you to their Block list.

Their device might have the wrong settings set up to be able to receive calls and notifications.