Are you guessing about the cordless phone? 2.4GHz phones were terrible, because they were vulnerable to severe interference from microwave ovens and other sources of microwave transmission. They were long ago superseded by 5.8GHz, which was subsequently superseded by 1.9GHz dedicated band DECT 6.0 phones. If you aren't using a DECT 6.0 phone now, recycle the phone you have now and join the 21st century.
There is nothing else that would contribute to "fluttering" sound quality on that fast of an ISP connection, unless you're running a crapload of video streaming, a torrent, or porno site.