Hiya Jon -
to the forum.
It's been a while since I needed to access my VM (then NTL) account from my Orange mobile, but it was possible. I find I can't from my O2 mobile however.
As far as I know you do need to be logged into a VM connection to access VM's POP3.
One way around it is to have your VM email forwarded to a GMail account and pick it up on your phone that way?