That could indicate that the SIM is faulty or damaged and not making contact with the device correctly. Best to ensure that the SIM is correctly inserted and test another SIM in the device to ensure the device is reading the SIM correctly. The SIM can also be tested in another device, but it depends on the device as well.
If the SIM is found to be faulty, it's best to call in to our support centre and order a replacement. If the device is faulty (and still under warranty) then the support centre can also arrange a repair or replacement.
If you're in the Joburg area, coming into our walk-in centre is probably the quickest solution.