Explanation
The best action for the network technician to take to prevent data loss in this scenario would be to execute a remote wipe on the lost cell phone. A remote wipe is a feature that allows the administrator to erase all the data from a device remotely. This will erase all the private company information from the lost phone, preventing it from falling into the wrong hands.
Other actions such as disabling the user account, locking the phone and turning off the service may not effectively prevent data loss. Disabling the user account will prevent the employee from accessing company resources, but it does not erase the data on the phone. Locking the phone may prevent unauthorized access, but it does not erase the data. Turning off the service may prevent the phone from connecting to the network, but it also does not erase the data.