Good Day All

Please advise me

I recently purchased 4 Snom D713's.  All of a sudden 2 of them started experiencing call quality issues. If you dial or receive a call on them, the line becomes scratchy and cuts, whenever the user on the other end is speaking. Both on the Load Speaker aswell as then handset

I then replaced 1 of them with a new one straight out of the box, it worked for about 30 minutes or so, then it started having the exact same issue.

ive changed Ethernet ports and interchanged between the different subnets. Still the issue persists.

Please assist