My internet connection stops working when we use the phone - how can this be

prevented? I have a wireless router box with TalkTalk
And we've put routers between the phone line and the internet line to separate them, if that makes sense
We've done that for both telephone lines in use, however not for the master socket as that's near the front door and we do not use it
BUT I've found that whenever someone uses the phone the internet stops working
And this is not very convenient :/

So any suggestions? I emailed TalkTalk a while ago but they have not emailed back
xx