The noise comes direct from the speaker not from the handset.
The test with the smt-i6010 was with the same extension number on the same poe port.

I have 5 other smt-i5243, all have the same noise, on normal workday i think this is not a issue.
its a litle bit special, that was placed a business phone in bedroom.

I will try later a smt-i5230