jsuresh:
Did you recently port port in a number to that GV account? Sometimes, the porting process causes the number to be unreachable from certain Telco Providers. For those calls that are going directly to voicemail, are they from a single provider such as AT&T? In cases like these, the owners of these numbers have to contact their provider and complain about the issue and normally, they have to update their "routing" to get rid of the problem. I don't think this issue has anything to do with a "firmware update", since some PSTN calls are coming through fine.
As for a Voice Mail that is other than GV, that can only be possible if you have another forwarding phone set up on your GV account with has its Voice Mail setup to answer in less than 25 seconds (~5 rings).