Noise forwarding calls from Grandstream HT503

I hear a noise (only me, receiver, not the caller), when I receive calls forwarded from ATA Grandstream HT503 through FXO port. What could be? Noise from DSL? But there is the filter for DSL. In which way I can avoid? Thanks

Hello Frank,

Since the noise is only heard by you and not the external caller I would imagine that is like missing parts of the audio or like an electronic artifact in the line or robotic noise. So please check the network where this HT503 is connected and check if you can use a codec that requires less bandwidth such as G729 or iLBC.

As you mentioned, also verify that the DSL filter is properly connected.

I checked. I have the same noise on port FXS of ATA Grandstream HT503 using an analog phone.
What can I do?