No reply usually means I am not free at the moment
It may mean I am doing experiment, and I don't want to contaminate my phone with my dirty gloves
It may mean I am watching movie
It may mean I am having a game of chess with Dr. Chow
No reply may mean my phone is on silent mode
I may have left it in my room while watching TV in the living room
It may have rang/vibrated when I was busy (sleeping/bathing/cooking/doing laundry,dishes, etc...)
It may have vibrated when I was walking/running (walking home,catching bus,etc)
No reply may mean I am not interested
It may mean I don't feel like going at the moment, yet last minute I might change my mind
It may mean I don't feel like going, yet I don't feel like rejecting you in the face
When you get No Reply from me, Don't worry, I will reply sooner or later. Unless I am really not interested.
If I get No Reply from you , I guess I shall assume the same too....