iPhone keeps saying call failed

Full bars but dropped calls usually means a voice over LTE problem, not signal. Go to Settings > Cellular > Cellular Data Options > Voice & Data and try switching between the LTE/5G options. Also turn Wi-Fi Calling off and back on. If that does nothing, Reset Network Settings (it forgets saved wifi passwords). After that it's the carrier side. A replacement SIM, or them reprovisioning the line, fixes a surprising number of these.