As a long time Sprint customer I am a thinking of leaving and going to ATT. It has better coverage where I live. But the one thing I love about Sprint is my google voice account having the same number. If I leave sprint and go to ATT will I be able to take that google voice number with me. If not that might be enough to keep me with sprint
The number you are using must exist on either Google Voice or on Sprint's network now, not both.. When you go to Google.com/voice and go to settings you will see your Google Voice number listed up top. If that is correct, then whenever you get a new phone on a new network you can simply have all calls to your Google Voice number forwarded to that phone, setting it up using the "Add another phone" option at the bottom of that GVoice settings page. This way your contacts keep in touch with you using the same number as before but you can have it ring on any phone you like, and likewise any Android phone can use the Google Voice app to make phone calls through Voice so it will appear as though you are calling from that number as well. Text messages can come through the GVoice app as well. The only thing is doesn't support is MMS.
This is how I am setup. Nobody actually has my T-mobile number or my apartment phone number, but when they call my Google number I have it set to ring on both my cell and my landline. It works very nicely.
If your number does not match your GVoice number, then it is your Sprint number, and you would have to port it into Google Voice (which will break your current contract/service) so that you can set it up as I explained.
Here is a thread which I used a few months back to set things up when I was leaving Verizon and going all in on Google Voice, a lot of good tips and instructions in here:
[GUIDE] Smartphone Service for just $30/month | Galaxy Nexus > XDA