I can think of two ways of doing it. The first is to put all the lines into a single table, with a column for each phone line.
That is, the feature list at first looks much the same as now, but with the checkboxes &c. on the far right instead of the far left, to the right of the prices. The "Phone line: One" message would also be at the far right, atop the column of checkboxes. When you use the spinner to add an extra line, it adds a new column of checkboxes on the right, with a "Phone line: Two" header. You might also consider putting the total price for each line at the bottom of that line's column, so that it updates as the user turns features on and off.
Getting the accessibility right for a complex-shaped form like that might be very tricky, but it'll be an important consideration.
The second alternative would be to have a separate page for the features on each line. The first page shows the spinner to select the number of lines, and optionally the form for the first line. After the features are input for the first line, and the user presses the "Next" button (I assume this is already a multi-stage form), the next page says "Line 2 of N" and shows the same form of features, for the user to select the features for the second line. This pattern repeats until all the lines have been entered.
If you go with a design like this, you might consider populating each line with the options selected from the previous line, if it's a common use-case to use the same features for several lines. (Using the previous line also makes it easier to have say 3 lines with lots of features, and 4 more lines with a smaller set of features.) If there are any features that are only available for the first line, or that automatically apply to every line, you should say this up-front on the first line's page and then display a reminder message to the user in the place where that feature would be on subsequent pages. In addition, if you do it this way, it's important to let the user revisit earlier pages, for when they get to the last page and realise they've spent too much money and want to turn off a feature for every line.
Additionally, if the most common use case is to have the same features on every line, you might make that case simpler by having two alternative forms. It would start with a simple form with the spinner, one list of features, and a button for "I want different features on each line." When that button is pressed, you can change the form to one of the more complex designs.