Most often, the “call failed” error appears when there is no Internet connection. Check whether your phone is connected to the Internet, if you are connected via Wi-Fi, check whether the router is working. If you cannot restore the connection yourself, ask your ISP for help.
Power saving mode
Some phones have a built-in power-saving mode. Your phone blocks calls when the battery is almost completely drained. If power saving mode is enabled, messenger functions will be restored after the battery is recharged.
Blacklist
Also, outgoing calls do not go through if the recipient has added you to the blacklist. No notifications are received by the blocked user. If you suspect that you are blacklisted try to call the recipient from another number and clear the situation up.
Other failures
Sometimes users simply can’t press the call button on Viber. This problem appears because of overheating, system failures or corrupt files. Reboot the phone. If the problem persists, uninstall the messenger, and download a new version of Viber.
