What I needed to do was read this
then order a new line.
Tell them you have an existing line
and a few clicks later is the all important option to:
They can port out the number from underneath a line, without ceasing the
broadband line. *I think* they can do this with the existing provider
holding onto the line, but if you're in some fixed term deal then it may
cause enough ructions that breaks the deal (early termination fees etc).
ISTR one issue is that your provider's billing is keyed on the phone number,
and so they need to handle it changing, which could cause a new account.
Also there are issues depending on who originally owned your number (fine if
BT/OR, trickier if NTL/TW/Virgin Media, hard if other VOIP)
Basically, you need to ask them directly if they can handle you circumstances - but
they can do it if everything is right. You don't need to take their
broadband to do it.