I do this. I don't mean to be rude, I just think we're done. Mainly I just say, 'OK bye' quickly and people are a bit taken aback that I'm gone.
I found out that my SiLs ring each other as some sort of support group after they've hiung up from me. My brothers have to intereven and explain I'm no good with good byes. I can't STAND long goodbyes at parties either. Drive me crazy. Just fricking LEAVE.
Maybe the real DP has been abducted by aliens & this is a replacement. They've got him fairly convincing, but overlooked this minor detail which will be their downfall. All the technology in the universe can't account for trivial human convention.
My ex used to do this, I remember one of our first long conversations when it was all lovely dovey, we talked for ages then he just said ok then and hung up, I was so sure the phone had been cut off by accident I called him back and then looked like a creepy stalker girl when he sounded bemused that I called him back!