keep getting error message “call failed”

Usually happens when your location does not have adequate cellular coverage from your carrier. In that case, only solution is to find a better spot from which to make the calls. You want the most bars next to the carrier name on top of the screen.

You can try turning Airplane Mode on (turns all the radios off) and off, so you force a resynchronization with the carrier. Might help sometimes.

Turn your iPhone off, wait for 10-15 seconds, then turn it back on and test it.

If the issue still persists, contact your carrier and ask them.

Regards

Giulio