Nothing legally binding happens over the phone (a verbal contract being worth the paper it's printed on, etc.).
Send me a letter. With a return address. Which I will verify.
As someone who HAS had court bailiffs knocking on my door (not looking for me, I hasten to add), they are infinitely more likely to turn up (and be happy for you to call police to verify identity, etc.) or send you a dozen letters first. Phone calls don't figure until after they've made first contact.
And emails? No. Let's not be silly.
Like all things, put your concern in writing and I'll deal with it, until then you're just some guy SAYING that I owe you money or I'm going to jail. Put it in writing and I can use it against you in a court of law (I imagine, and I have a reasonable way to deal with the situation in my own time. Until it's on paper, it doesn't matter what you promised me, what you say I owe you, or anything else. And the guys who come to your door bring paper if you're GENUINELY that elusive that you've missed months of communications, and are very understanding when you ask if it's okay to call the police to verify their identity and authority.
As someone who's invited companies to sue my person on several occasions, even offering to initiate in small claims on their behalf if necessary, I have yet to have anyone actually take me up on the practice once things are put in writing. Especially not the insurance company that tried to retroactively cancel my insurance and were mistaken to think I wouldn't keep the still-postmarked letter describing those actions, and another which said I "had nothing more to pay" quite clearly - letter which strangely they "could find no trace of in their system" until I was able to furnish them with copies.
However, I think it must have jogged their memory slightly as they switched from "We're going to take you to court" to "Please don't take us to court or report it to the financial authorities, here have some compensation".
Get it on paper. Until then, it literally DOES NOT EXIST.