afaik the phoneline is for interactive services, some tv shows, such as "test the nation" etc have a real time voting service, some interactive games dont work if the phone is unplugged, there is a cost to the service i believe so most people dont bother, disconect it to stop the kids accidentaly running up a bill, or dont want the inconvinience, if your due a sky visit just plug it in and unplug when he leaves.
Updates are via the dish, these can be forced by unplugging, press and hold the backup button when powering back up until all the coloured lights come on, takes 10 mins or so to complete, 99% sure the update will work reguardless of phone line..,
Sky advertising free sky plus hd boxs iirc, may just be to existing customers though, and there is probably a £35 instalation fee.