why it happens only in one direction? it should affect both right?
It will not show up on a voip to voip call, It shows up in a voip to Digital world.
Were you go from outbound on a Pri, DS-1, E-1 then it is a digital signal.
On the other end were it is being converted A/d in the telco world it is allways Grounded.
There is nothing in the telco word that would ever be ungrounded because they already had these issues years ago.
But Ground issues can come across and noise, they can effect one side or both and they come and go and can change right in the middle of troubleshooting which is why they are so aggravating.