your using part of your unlimited data plan....
now if you make phone calls using google voice it goes against your plan minutes unless you have it setup to use wifi only.
The work around is dialing your GV number, and then punch your VM code, then press 2 to make a call, then dial the number you want to call. It will then "route" through your GV number and not go against your minutes. I do this for those calls that I know may be lengthy. I just have the full sequence and pauses in the contact info.
As for the texting, I believe that just using the GV number will still use your sms counts. AND, no MMS with the GV number.