Umm FWIW..
Broadband for BT Sky and most everyone else is delivered over a copper cable or line excepting Virgin media .
That line carries the voice band channel ie normal phone calls, but the broadand is carried at much higher frequncies hence the need for that ADSL filter.
Bog standard ADSL orignates at your local telephone exchange and the performance is dictated by the distance from said exchange and the quality of the wiring from said exchange, allumimium cable can be shite poor..
VDSL OTOH often refered to as Fibre is where a fibre cable carries the broadband to the cabinet in the road hence the term FTTC (Fibre to the cabinet) therfore for a lot of people that is closer then the exchange so higher speeds can be relasied usually up to 80 Megabits however that signal falls of more with distance but as the cabinet is closer the performance is usally better. It alos euses the copper pair cable and that cariries the phone landline signal.
Now to make it a LOT better we have FTTP or Fibre to the premises this can go up to the Gigibit region but the roll out isnt that good as yet owing to the the expense of installing it but some indie suppliers are doing that their not to do with Virgin or BT firms such as Gigaclear and the like
Virgin media do it slighty differnt. They use their version of Fibre to the cabinet BUT they use a TV type co-aixial cable which can work better then copper wire. Oddley enough they do have a simple copper cable to deliver landline phone on!. The Co-ax cable part also carries their TV service.
Then theres such as VoIP, Voice over internet protocol. I use this it meands my Landline number comes up on the mobile phone by a programe called VOIPER its not simply forward or diverted but uses the phone data chanel to work on, best of both worlds was phoneing someone in the states the other night for around 1 penny a miniute clear as a bell better then the normal phone.
As to why phone coverage is poor?
It costs to install mobile phone base stations so they only use what they think they can get away with often people object to them on various grounds. Their the main reasons.