If they had VOIP, then turning off the router would have disabled the home telephone.
In the crimescene photo below, on the edge of the top of the short shelf in the office, there is a small box with a blue wire plugged into it.. that looks like the router used for some VOIP phone services (which is secondary to the main router providing internet connectivity to the home)... immediately distinguishable by those who might also have them. I don't see a phone anywhere. These days, people often don't even bother having a 'home phone' any more, and simply rely on their cellphones.
I doubt if the disconnection of the modem/router (if done by the perp/accused) was for the purpose of cutting off the phone service, but rather, to cut off the possibility of IT forensics personnel being able to establish connection/disconnection times of certain devices, and possibly MAC ID info regarding which devices connected, and when, etc. Interesting that they are not able to establish a more definite time of when the modem/router was disconnected (a span of approx 6 hours during which this could have been done??). (Wouldn't the internet provider be able to tell when that modem/router was put offline?)
https://pbs.twimg.com/media/C2kG0nLUoAAdrP8.jpg:large