how olds your phone? was it doing that before? or did it just start? it might be a signal problem in your area. the first moment i had did the same thing and i returned it same day and the second one did the same thing found out it was a signal issue in my area. took a few hour but it fixed itself. only happened once in the last 2 months and reseting my phone fixed it.
well the phone is about a month old.....sprint tech at the store told me it was probably a software problem with one of the apps and that i should hard reset.......so i reluctantly did and it's fine now......not really sure what the problem was since i only had the phone a month but if it happens again i'm raising holy hell