I use several techniques, depending on how much time I want to spend.
I'm good with voices. So once it's clear it's a scammer - there's often a little click before the message begins - I repeat "Hello? Hello?? Hello?? I cain't hear you..." over and over again, sounding like an old country fellow about 115 years old. I finally end with "I cain't hear you....good-byyyyeee", in a fade-away voice.
If I have more time, I use a similar old country-fellow voice, but with great enthusiasm and very poor grammar. I recruit them for a small foreign charity (which really exists; every word I say about it is true) that helps orphans:
"Oh, I've jes' been a-waitin' fer someone to call, so's I kin testy-fy about (name of charity). You know, they hep them pore little orphan cheeldren, way over thar in (name of country), and I jes' know you wont to do the Lord's Work and wont to hep. I got some materi-el about it rite cheer and I kin send it rite to ye, if ye'll jes' tell me whar to mail hit and send me a little sumpin' back for them pore little orphans".
I can testy-fy ad infinitum, rattling it off as fast as I can, all about them pore little orphan cheeldren. But it usually shoos off the caller within 90 seconds.
For a while, scammers would invariably mispronounce my name and start off "Mr. CraigBreek?? How are you today?"
I'd reply, "Not very well", in a die-away voice.
"Would there be a better time to call?"
"No, there won't be a better time...".
"Is there anyone else there I can talk to?"
"No, I'm all by myself, but the ambulance is coming soon..."
If they disturb my day, I figure I might as well have a little fun out of it.