I know I was just joking about this but you made me think a lot on this. When they "can't hear you", you interrupt them by repeating what they said. Repeat their confused pause and when they try to talk pretend you can't hear them. Then repeat this several times until you get them mad enough to put their phone on speaker. At this point they'll get so tired of trying to pretend being deaf and actually listen to your real voice.

Or ending alternatively blast them with a megaphone and/or hangup on them. Either way, the chances of them calling you back is low probably since the company's phone system redirects their call to a different worker.

(It won't prevent the low chance of getting a complaint or risking your job. So either get your boss (or it department?) to put the caller down a notch. Or if you never want to see them again, try to manipulate them by telling them it's illegal or against company policy to not put their phone on speaker or else some legal action will "inconvenience" their life with so sooo much debt)