On my side, I am a residential customer [actually my parents] using Comcast with a Motorola Surfboard SB6121.
Comcast > SB6121 > ERX/ETH0 > ETH2 > LineLink ATA > DECT phone
Let us know if changing out the modem helps.
The best answers I found:
https://forum.fortinet.com/tm.aspx?m=119616
https://www.reddit.com/r/networking/comments/20j2bi/comcast_leaking_internal_dscp_cs1_to_customer/
What I'm curious about is if changing DSCP from CS1 to CS0 on the LAN side will actually help. As goldlineIT found, it does effect Wi-Fi since it has it's own QOS/WMM.
Also wondering if I were using a Comcast branded modem, would DSCP values be different, but I have no way to test.