Google Voice, someone break it down for me

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http://androidforums.com – Here is my situation... I've bought a Nexus 5 for myself and looking for a Nexus 4 for the wife. We are on Sprint, with one phone out of contract and the other paying an early termination to get out. I'm planning on taking both of these to T Mobile $30 prepaid. Now, we both use more than 100 minutes a month. She is about 400 and I'm about 300. Therefore, I'm trying to figure out this whole Google Voice/Groove IP thing. I made a free Google Voice account and got a number. From there, I'm lost on how to use it. We both would just want to keep our cell numbers and use Google Voice for m (General)