You’re missing out on a golden opportunity, here: when he answers the phone, go over prices for things like exorcism, interventions, advice, etc. Good little way to supplement your income.
My SO got a new number probably five years ago as he’d moved from Chicago to the Seattle area and wanted a local area code. He still gets collection calls for a *couple *different people.
Somewhat tangential but since I like to bitch about it: I’ve had my number since 2003. It’s one digit off from a neighboring city’s HR department. I get calls all the time from people wanting a job and potential employers wanting to confirm employment. I used to call them all back but gave that up. My favorite was from the guy who wanted to apply for a job as a crane operator. Dude! You can’t even dial the phone! 
Someone has been giving out one of my (unlisted*) work numbers for years (at least 10) and we get collections calls all the time. Sometimes I play the ‘sorry, wrong number’ card, sometimes it works, usually not. Sometimes I nicely tell them there’s no one here by that name or that they just plain have the wrong number and play nice while I ask them to remove the number from the database, again, sometimes it works, sometimes it doesn’t. Sometimes I tell them I’m going to report them to the FTC/FCC/etc (whatever initials pop into my head), same thing, doesn’t usually stop them. More often then not, when they ask for him and I say ‘who’s calling’/‘what’s your phone number’/what’s your name/you know I’ve asked you to stop calling X times now/etc, I tend to get hung up on before I get get enough info to actually report them.
Part of them problem is that he’s still clearly a deadbeat and still clearly racking up debt so it’s not like it’s just one agency calling, it’s lots of them. The last agency that called threatened “me” with arrest which I know is a huge violation (all I did was tell them it’s the wrong number).
I always forget to try the ‘from this point on, please only contact me in writing’ thing to see if that knocks down some of the calls.
Since this happens so often to me, I’ve had some thoughts on ways to stop it, but none of them are very elegant and involve a ton of government oversight and a lot of expense for collection agencies.
Here’s an idea that might work better. Collection agencies just mark down that the person said it’s the wrong number, that gets reported to the credit bureaus. Either just that it’s a bad number or maybe how many times it’s been tried and failed. When you go to get more credit, lenders will see that and, maybe, choose to check the phone number you’ve given them to make sure it’s correct. They call, your cell rings (or the person at the other end verifies that the phone belongs to you) and the lending process continues. OTOH, the lender calls and someone screams into the phone ‘stop calling for him, there’s no one here with this name’ or otherwise doesn’t confirm it) and the lender can do what he wants with that info (deny it, give the applicant a chance to correct it etc).
The collection agencies could also then update their files with a newly ‘verified’ number and remove the old one.
You want to get MORE credit, let’s use your phone number instead of mine.
*this is important (kinda) because sometimes they’ll say ‘maybe he worked there in the past’ and I’ll respond with 'no, I’ve been here for a long time and even if he did, no one knows this number and it’s not listed anywhere, not in the phone book, not on the internet, nowhere.
I also had this problem with the additional wrinkle that I would get texts saying things like, “Hey, man! You’re sister is out of the hospital, thought you’d like to know.” And me, being a considerate person, would respond to let them know they had the wrong number. Turns out it was likely collectors trying to confirm mine was a working number because the collection calls would increase after that. Seven or so collection calls a day, several bullshit texts a day. Honestly, the only calls and texts I got for awhile were collections.
Finally I just changed my number.
On a similar note, someone used my email to sign up for fantasy football this year and so I got all their notifications. So I went to the site and went through the forgot password process and took over the team and changed the name to “I stole this team from X”. I won.
They will not believe you, and they will call more often once you confirm it is an active number.