The Librem 5 needs a 3G (or 2G, which no longer exists on AT&T) connection for calling, due to the modem’s current lack of VoLTE-capability. Setting it to 4G only will automatically ensure that no calls can be made. (Until and unless the VoLTE issue gets sorted out.)
(Note that some countries/carriers may not be requiring VoLTE yet, but the U.S. carriers do, or will soon.)