The phones runs at the top of the 5G band.
I suspect since you are running at 2.4 since a lot of your equipment does not support the 5g band. It is unlikely the phone is interfering with your network.
The red light means the modem has lost connection to the main office. This will causes packet loss and errors. The ISP should also see log messages on their end so hopefully you can convince them to fix this.
I am having intermittent issues with my wireless devices(MacBook Pro, iPad, and iPhones) while using wireless connectivity. I have always heard the cordless telephones can interfere with the RF from the wireless router. Is this really true? I have AT&T cordless 6.0 Dect phones.