[mothered]

Is a phone call with the support the most likely way to ensure this goes smoothly?

Phone communication Is Instant and anything you've said cannot be taken back, so be prepared to answer all questions accordingly.

And what should I say?

Upon opening the box, the phone was missing.

I was thinking to say the package arrived, but when I opened the iphone box, only the charger and manual were in there, no phone. Is this a plausible story?

Don't go Into anymore detail than you need to.

As said, tell them the phone was not enclosed In the box and take It from there.

And if they ask me if the box was sealed or otherwise tampered with, I should say it was not tampered with and was sealed, correct?

Say that you didn't notice at the time of opening the box, but will check and get back to them.

If they decline the claim on a perfectly sealed box, It serves as a backup, thereby you'd then say It was tampered with.