Landline cuts in and out not solved

We have the ARRIS modem setup where your phone directly plugs into the back. The past 7 month people calling my coxline line cant hear us because the phone cut in and out.         Today we received a new ARRIS modem and it is installed correctly and the cordless phone still cut in and out.

What should be my next step to check before purchasing a brand new cordless phone system, that may not even solve the problem????