telephone and internet together

[nextqueen]

Following on from my recent questions re problems with multiple handsets. if the signal from the base unit isnt reaching our handset on the third/top floor of our townhouse, can i instead buy a single telephone to put in the phone socket that already has the modem plugged into it. it seems to be plugged in with a small white box that is a filter which has the cable plugged into it and another hole for a phone line. i did try this with a cheaper phone a few months ago and it stopped the computer from working. why was this and how can i put this right? Starting to lose my patience again with this new house and telephones, had so many problems with BT itself when first moved in, grrrr!

[boxtops]

Can't you move the base unit? we have a 4-storey house and the base unit is on the first floor, with phones on ground floor, first floor and second floor. That works fine.

[boxtops]

PS - the modem and base unit are plugged into a phone adaptor in the same phone socket. We're with BT.