Hi
Well, either you have poor or intermittent reception - if its bad when the call is made the network may just send to voicemail if "forward when unreachable" is set to divert to voicemail.
Check here:
Open phone dialer
menu button
call settings
call forwarding
voice call
If "always forward" is set, that would cause all calls to go to voicemail! Note that you do need a phone network connection to be able to change these setttings
Hope this helps
Roger