[MN] Two modems on phone service!

Hi I just got Mediacom phone service yesterday. I order my own modem a arris tm822g docs 3. My old modem is motorola sb6121 docs 3 as well. When the tech came to hook the phone modem he left my motorola modem for internet access and just hooked up the arris modem for phone service. He used a splitter to hook both modem together. My services are working fine. Do you think I should just use the arris modem for phone and internet or is better How the tech has it install. I download a lot and don't want to have interference on the phone if i am downloading. Thanks for the help.