I used to have 5 lines with One Tel. Then they were bought out by Talk talk.
I now have no services with Talk Talk and would never, ever, use carphone warehouse (who own them) for any business or residential services. Their customer services is truly awful, and they fully deserve to be deserted by their customers and go down.
We now use BT for the line rental and buy the call packages in: we are also moving to SIP (Voice-over-IP) trunk lines instead of analogue lines, which work over the broadband link.
My opinion: don't touch Talk Talk with a barge pole!
BW