Certainy sounds as if BT might be best. I have been looking at alternative numbers for using with my BT line (which I have kept) that you dial and divert the call through them. They are cheaper for calls altogether , but , impotantly for me, calles to Australia are only 0.5p a minute. So I might investigate going to BT and then using one of these alternative numbers for calls.
Has anyone used them and can recommend any please? One I looked at was 01815 (or similar) I think and sounded good. Janet