Thank you very much for your information.
@antric The cordless Philips phone is from my grandmother and is at least 18 years old…
Don’t think it’s certified to the CAT-iq 2.0 standard.
@Walter_Wp I registered the Philips handset with IB Plus because the input of the fiber optic cable and the network cable for distribution in the apartment are housed in the electrical cabinet. The electrical cabinet is housed in the cleaning cabinet. So I would have had to put the base station of the handset in the cleaning cupboard.
So I have no choice but to live with it or buy a new compatible handheld device.