Sunny,
In most cases, a disconnect with cause 16 (especially if there is no connect time) usually means that either there is too long a PDD from your PSTN provider or possibly a problem with no voice in one direction.
The low duration calls could be due to dialing wrong number (or having wrong configuration to cause wrong number to be dialed) to the PSTN and a recording (“The number you have dialed is incorrect…”) being heard. In this case the call will show connected, but the caller will hang up the phone as soon as the recording is heard.
MikeM
mike_voip@hotmail.com