Lets assume this Network topology
Let assume that Extension A register with Site A and Extension B and Extension C register with Site B . and also Extension C configured to pickup calls from Extension B .
when the call come from any extension of site A to extension B or Extension C the call pass normally but when Ext. A try to call Ext B and Ext C try to pick up the call then the call Drop and Ext C can't hear any sound from Ext. A and also Ext. A hear Echo .
after checking we found the customer has another sites and when any call come from any site expect Site A , Ext. C can pick up the call normally but the issue is with Site A only .
As it appear the problem is with Media so :
and this is Bug in the version V200R003C00SPC300B013 so the solution was to upgarde U1960 to V200R003C20SPCb00
the solution is Upgarde to this version V200R003C20SPCb00