If you're not bothered by having a conversation with the homeless, indigent, or hard-up, then it's more doable than you think. You're not just subject to the chance of happening upon someone already in the business of providing these services. You can be a job creator.
With mandatory (and otherwise widespread) masking policies right now, it's even easier than under normal circumstances.
At least where I live, there is simply no option for voice/SMS SIM cards other than a subscription drawing from your credit card or bank account etc. So it would take the proxy person to also hold a bank account or credit card and be on the hook for any charges to the SIM card. Even with government ID etc they can't just go to a shop and pick up a prepaid for cash.
Theoretically possible, yes, but in practice, I think the only option is if you have the "right" connections with organized crime. I don't think you'd be able to solicit on the streets, even in the capital.
"desirable illegal thing isn't available on the black market" is wrong. Not even worth saying it. No, that horrific thing you're thinking of isn't a counterexample but you probably can't afford it.