Threatening Call on a Home Phone

The second time it happened in the same apartment, I was already 9 years old. We had three home phones at home (this is important), one corded in the hallway and two, at that time, new — cordless, all of them connected to the same network. For those who didn't catch that era, let me explain, you could talk on one phone, pick up the second and eavesdrop on the whole conversation. Now to the story. It was evening, my parents were getting ready to go visit someone, they left me a pot of buckwheat with sausages, said they wouldn't be long, and left. I locked the door behind them and the door to the room (I was still scared of that old woman). Turned on the TV. After a while the phone rings. A male voice:
— Hello, is this Seryog? Seryoga? (I'm Sergey Sergeyevich)
— No, dad's not home, he left
— Seryoga, I know it's you, open up
I hung up. There was no creepiness, I thought someone had come for dad. I wanted to call him on his mobile. I press the call button to dial the number, and the guy is already yelling:
— I know you're there behind the door, open up, come on, open the door!!!
I hang up the phone and realize the situation is fucked — running away isn't an option (I'd have to open the door), I can't call my parents either (I didn't have a cell phone, and the guy might hear me on the phone). And what do I do? With tears in my eyes, trembling hands, I start eating my buckwheat from the pot. I shook for about an hour more, then I think I went to sleep. That's how it is.