The news release
seems fairly clear that the line rental price freeze is for all residential and small business customers, not just those with a line and broadband package.
Of course, the price freeze does not stop Zen putting up the broadband charge, call charges or the cost of add-on services. Until now, many companies including Zen have been loath to increase their broadband charges, but have had regular increases in line rental even though the wholesale costs have barely changed. Once the new advertising rules are in place, it won't be possible to 'hide' price increases from the headline figure for bundles by hiding them in a small print line rental charge.
I am not dismissing the value of Zen's offer; I'm merely observing that I expect them to step away from their historic reluctance to increase broadband charges as a result of committing to no line rental increases until 2020.