I reply quickly if I actually have my phone on me and notice it.
It's kind of a vicious cycle, I don't get many people texting or calling me, so I leave my phone somewhere lying around because I'm tired of carrying it when it's not serving a purpose. Then someone will actually try texting or calling me and I'll miss it and don't respond for awhile which leads them to not want to contact me again while I'm sitting there trying to carry my phone around constantly so I don't miss them again.
To tell you the truth though, I like to get lost in what I'm doing a lot and having my phone right there distracts my concentration. I much prefer to only have it around when I'm in the in betweens of doing things on my own. When I'm doing something and someone bursts in, I lose my train of thought and they take me out of the zone, this is what happens when my phone goes off whilst I'm working and I prefer to avoid it. There are times that I'm in social mode and if you contact me then, you're golden, but other than that eh... Maybe she doesn't know how to respond well, I prefer to show my true feelings over talking about them, or maybe she is having trouble getting alone time and is overwhelmed by all the people interacting with her.
We need time to think and process what people say and to create a good response.