score:10
Since the foreseen delay is more than 5 hours, the EU air passenger rights regulation (article 8(1)(a), as referenced by article 6(1)(c)(iii)) entitles you to forgo the trip entirely and instead get a full refund of the price you paid for the tickets.
You do not need to specify a reason for wanting to cancel, other than pointing to the change.
As a practical matter, sending an email to their customer service department is as likely to be effective as it is to keep calling.
If the change had delayed you for less than 5 hours, it would lead you into the fun question of whether it should be considered a "cancellation" or "delay" according to the regulations. I think even then the legal precedent would be in your favor, especially in light of the different flight number. But in this case you don't need to care about that, because either way you'd have an absolute right to a refund.