Can someone explain the causes of silent calls and possible solutions?
Check the transmission connection
The vendor is ZTE
In case if it is one site
More than one site and it happens intermittently
On Zte its very rare you get mute call. If it is you can check packet loss, any issue of transmission.
Most probably transmission issues
Can transmission issues like high BER cause it?
Yes, they can
Or faulty stm
And on what scenarios these happens?
At times one way at times 2 way
is this GSM?
Yes it is GSM
Are those mute calls made within the home network or are made inter-network?
I think yon need to check is the AMR codec mode sets are aligned.
It is also possible that there is some problem on MGW side, so ask CN guys to help investigate this issue.