Yeah, calling is the best, for sure. We need to keep up the pressure.Particularly calling them if you have the time to wait on hold - tying up their call centres so they can't deal with routine business calls is going to have way more of an impact than sending an email they can set up a filter to trash. Physical mail would also work, particularly if you pay for registered mail that an employee has to sign for.
If you do call, bear in mind that the person you're talking to is not the enemy - if anything, they're probably on our side - so be polite and friendly, but do also try to drag out the call as much as possible to maximise the impact. Part of the idea is to make a denial of service attack on their phone lines.