If you are facing that call failed iphone issue again and again. I know it can be very frustrating when you are making some urgent phone call and then your phone says call failed. But don't worry here are the steps by which you can fix your issue:

Dial *#31#
Turn off and on Airplane Mode
Restart your iPhone
Remove and reinsert SIM card
Reset Network Settings on iPhone
Reset All Settings
Update your iPhone’s software
Check blocked contacts list
See if call forwarding is turned on
Contact your carrier