To fix this problem you may first clean the Sim card pin connector, then check for possible damaged. Replace if necessary. If the connector already cleaned and seems nothing wrong on it, refer to the solution below.
Nokia 6124c SIM Card Connection Line Paths
You may start checking and tracing the VSIM and Ground pin connection first.
You may notice that the connection line paths of the SIM card pin connector is directly connected to the AVILMA IC, except for the VSIM and Ground pins.
If the SIM supply voltage and ground pis were both okay but the problem still exist, the problem may possibly lies on a faulty AVILMA chip. Try to reheat or rework the chip these option may only fix and solved the problem.