no internet with phones

Hi

Have a customer who has had fibre installed with 2deg

I think they may be using voip or they have their phone line plugged into the 2deg router.

All the lines on their modem are on. They did have internet, but once 2deg got the phone working the internet stopped working.2Deg refuse to do anything about the issue and claimed everything is working at their end.

I'm having a look at the setup today but wondering if someone could suggest why the phones would interfere with the internet.

Thanks

Ford