Are you sure you don't have any misbehaving spam detectors? The voice circuits are VERY robust, low bandwidth, and cellular providers prioritize voice calls. There's just no reason to purposefully throttle the voice calls, the bandwidth savings would be negligible.
But it's possible that your cellular provider has misconfigured STIR/SHAKEN validation and/or some badly behaving spam call detection.
But it's possible that your cellular provider has misconfigured STIR/SHAKEN validation and/or some badly behaving spam call detection.