We left BT a few months ago now (sort of) and joined PlusNet for broadband (hence the sort of) and then Soho66 for the phoneline (VOIP). We've got the Multi 5 with 3 phones in our single office. We are saving money and enjoying the extra features of VOIP - would recommend.
I can whole heartedly recommend 'business phone calls, based in Woodborough, Nottingham. Seriously competitive and really helpful if anything goes wrong. I've been with them for a couple of years and know people that have used them for a long time.