Fax Machine Interfering With Internet Connection

[Mystic Mungis]

My mum asked me if I could have a look at the fax machine in her office, currently there is a microfilter plugged into the telephone line which has a router and the fax machine connected to it, at first glance I thought this should work okay but then she showed me that whenever the fax machine is in use the internet connection drops.

I quickly googled it and I found a video which, as far as I could tell because it wasn't very informative, said that an rj11 splitter should be plugged into the back of the Line port of the fax machine and then the phone line and the router should be plugged into the other ports in the splitter. Luckily I had the splitter and cables lying around so I gave it a try and the fax machine worked fine but the router wasn't getting a connection, I tried every combination possible but still no luck. Eventually I gave up and just set everything back up the way it was and thought I'd ask here.

They have BT Total Business Broadband if that helps, thanks in advance!