Dodgy phone line on broadband

[Cassis]

We've had broadband Internet and telephone services with Club Internet since ADSL phone lines came to our village last June.  Recently we have been finding that after a minute or so on the phone, the line goes very faint and we can barely hear the caller and vice versa.  Anyone know enough about this kind of thing to tell us whether this is more likely to be a phone line or an Internet connection problem?  Or could it be connected to the fact that the phone line from the exterior junction box travels about 40 metres before it arrives at the phone point?

Phil