The connection isn't always torn down immediately. Different switches behave differently in this regard. I remember a long time ago being trolled by a friend of mine who refused to hang up. I wanted to call someone else, but every time I picked up the handset to dial out, he was still on the line laughing at me.

So if you're served by a switch that operates this way, the scammer just holds the line open, plays dialtone and ringback tones appropriately, and you're none the wiser.