The above idea is interesting - is there any lag/delay/failed calls with doing this. Overall service is pretty good?
Does the caller ID still work so you who is calling?
Google Voice call quality can fluctuate wildly. Basically, it's worth exactly the price you pay for it, and it's limited to US numbers only.
CID can still be passed through in setups like this. One interesting thing of note is that if you're willing to pay for the additional cost of a proper VoIP provider, if you select the right one, you can get all the same features of GV and more.
My first question to you is who is your current cellular provider? Are these robocallers leaving voicemail messages and are they charging you for voicemail time, even if you don't listen to them? The fact that they're clipping you for unanswered incoming is kinda jacked up.
Given the unanswered calls are killing your balance, I'm guessing you're not using a lot of mobile minutes in a month. I want to say you're US based, but a part of me is thinking you're Canadian... apologies for not remembering. If you're Stateside, a better and more reliable option than GV might be porting your cell number to VOIPo's cloud VOIP service: $3/month for 500 incoming minutes, SMS support, added FAX support, etc. The only thing you wouldn't be able to do is make calls back out with your current number on this setup showing up on other people's CID display when you call. There's ways around that, but they would add cost. Alternately, you could step up to enchantment with VOIPo and get the full-blown package with the same features for a little more than twice the price after taxes. That would also let you receive and make calls at home without wasting cell minutes and have at least some outbound calling show up as the proper CID number, and give you the option of setting up a softphone for use with the service if you're using a smartphone.
There's more convoluted VoIP tricks that could be applied, but honestly, the best solution to your problem reaches a bit beyond blocking robocallers by porting to a VoIP number... it's an effective way to gut the crap calls, but the real problem is the fact that
you're being charged for unanswered incoming calls. Your best solution is to find an alternate carrier who isn't going to do that.