Forget about new line for now, try broadband on the original line, which I presume has the phone number on it that they want on it too.
If that BB is vastly slower than what they go on the other line, then this will cost more, but avoids the number migration issues.
i.e. leave the current phone number in place...
re-activate the second line with whatever number they want, get the bb running on it.
Yes means paying two line rentals, but can be useful for business, as with the number of times you have to hold these days, I do a incoming/outgoing line trick, i.e. one line has a minimal inclusive calls package, the other has totally inclusive.
I also run BB on both phone lines, to give me back up for the days when one is mucking about. Need to use two different providers for that.