How many signal bars are showing on the screen when you think that you have missed calls etc?
If you're in a bad reception area i.e no signal bars, the phone may lose contact with the phone tower and if the phone can't be contacted the calls will go to voicemail. You shouldn’t be able to call out either if this occurs.
This can sometimes also happen when in buildings or in underground carparks etc. anything that can reduce the amount of signal being sent/received by the phone.