Hey Keletso, this may be owing to a buggy 'Dialer/Phone" app. Navigate to Settings > Apps > All > Dialer/Phone > clear data & cache... You will then retry taking apps calls and observe. If the issue persist, then it may be a issue with a defect battery of just a loose connection as the calls requires more power than when the tab is on stand-by. I once disassembled a similar model for to replace a charging point but it could be over 4 months and hence I do not clear recall if the battery cable was soldered onto the main board or is used a connector/plug. Let us know if clearing cache does not work.