In order to have an iPhone or iPad 'ring,' Zaurus is dependent on the Apple Push Notification service (APNs) from Apple. These pushnotifications should work through both 4G and wifi. It is possible that a firewall on the network blocks these notifications from reaching the device.
The iDevice sends a 'keep alive' out to Apple every 15 to 30 minutes. This signal is used to prevent devices along the route from closing connections. However, sometimes power saving modes may delay this signal or stop it entirely. If this happens, no notification will reach the iDevice anymore.
Turn off the power saving mode or restart your device and put it on a charger to resend the signal.