This post holds for Nokia 6720 classic Insert Sim Card problem issues most significantly if the device suffered water damaged, this solution may possibly help the problem.
If the device won't detect any sim card being inserted to it, try to clean the SIM card connector pin first. Just refer to the solution below for further troubleshooting.
Nokia 6720c insert sim card problem solution
You may first check the SIM card supply voltage (VSIM) and ground connector pins line paths. If both were all okay, the next option will be by reworking the GAZOO chip or replace if necessary.
