We got the same thing here with Alcatel, need to check your TCU (TCIF):
1. make sure you have the last version of TCIF unless you would need to upgrde
2. check TCU card by listening on each card for Silent call, it might take some time but it works
3. We also noticed here some card reconfigured with MU-law instead of A-Law, check this as well
Sometimes its TRX problem but most of the time it comes from TCU if the problem of silent call is happening anywhere/anytime.
If 3G to 3G is giving trouble as well, then it might be a wrong config of NGN.