I use Viophone (
http://www.voipfone.co.uk/).
As I said I took the free number they offered since one of the reasons for abandoning a landline (in addition to the high standing charge and few calls made) was the number of junk calls I was getting and have had none since the change.
This is the unit I purchased to be able to use my existing DECT phones -
http://www.voipfone.co.uk/shop.php?method=view&pid=12
One thing I especially like is that any voicemail left is emailed to me so as an .mp3 file so I can pick it up and listed on my mobile, tablet or any PC even whilst abroad.