First let me say that I have GoogleVoice numbers and use them. With that said...
Is there specific reasons you want a GV number? Perhaps for the way they handle calls or because it is free? Those are great reasons, but here is my take on it. To port in a landline number, as Highgrade mentioned, is a multi-step process, sometimes with problems. There is also a cost to port.
You could port the number for free to
Voip.Ms and fund that account with $25. Although you would pay for calls and them hosting that number each month costs a bit of money, I'm betting that $25 would last you many, many months.
Obis are great to access GoogleVoice numbers, but as I say, there are issues that may crop up that make the process less than easy. One is that if you don't answer the call or have an answering machine pick up fast enough, the call WILL go to Google voicemail. Some people like that function and some do all they can to avoid it.
There are other Voip services also that may or may not do what you want for a reasonable or free cost. So look at all options and decide which service works best for you. Perhaps it is GoogleVoice after all.