I still have my landline with BT and will continue to do so,as I think if/when you develop a landline fault it is easier getting it sorted being a BT customer.
I also am unsure about this and, like you, fear that moving my line rental to another company like Talk Talk may result in more problems should I encounter phone line problems in the future?
There's a rumour that O2 may offer phone packages in the near future so I'm unsure whether to go ahead with this to save money?
I
think that even if I was with Talk Talk (for my line rental), BT Openreach would still service my phone line?
I've been online for 9 years and have only been with 3 ISP's - AOL, Zen and now O2 LLU.
My personal experience of them is:
AOL - dire customer service and they don't have a clue about UK ADSL so a very frustrating experience
(this was the reason why I migrated to Zen after hearing about how good their customer service is!)
Zen and O2 LLU - first class customer service.