What diffrences bewteen dectphone connected to modem and connected to router (the tel indicator solid even no phone plugged in) . I noticed that some user dectphone is connected to router and some is connected to the modem.

I also noticed that unifi fiber user dectphone is connected to the huawei modem (ONT) and Maxis fiber user's dectphone is connected tp the tp link (ArcherC5v) router.

May i know what diffrences bewteen connect to router and connect to modem. Is it set by the ISP or we can change ourself?