Sounds like it's your cell phone or reception problem. Your GV # number is doing its part by forwarding the call to your cell. If you're not in reception area, then it gets forwarded to voicemail. It's only GV's problem when people (or yourself to test) call GV # and gets something different than dialing your cell # directly.
Also a way to troubleshoot things is look in the online GV account and click the 'missed' and 'received' call log.