That's a very good question. We could certainly use this slang for someone lying on the couch playing computer games or on their phone. It wouldn't be correct if someone was sat at a desk using the PC, or using the phone while walking or at a restaurant. It's really just a lazy person sitting on a sofa!
I like your phrase too. People spend so much time on their phones these days that sometimes the forget the people next to them! I have seen 4 people in a restaurant together spending all their time on their phones. I'm not sure what slang we would use for people like this, can you think of anything?
This is a new habit. How about a phone head? An i-addict?
I can't think of one that is commonly used!
In Vietnam, we also call them like that. (It is used by students).