First, are you familiar with phone number portability? That's the function that lets you take a phone number assigned to you (be it your landline number, cellphone, etc) and "take it with you" when you change carriers. For example, if I get 555-555-1234 from Verizon, then change to Sprint, I can take my number with me to Sprint so I don't have to tell the world my number has changed.
Second, Google Voice allows you to get a phone number that anyone can call. You can have that number ring through to any other phone. For example, I sign up with Google Voice and get 555-123-4567 as my "google voice" number. I configure my Google Voice account to forward any call to that number to my personal cellphone (555-123-4567). Or, I can have it dump the call directly to voicemail without ringing another phone anywhere. In addition, I can have it send me an email telling me someone called and left a voice mail. That way, I don't have to deal with another phone. Also, Google Voice can translate the voice mail to text and send that to me as an email.
Clear so far?
What I did was a combination of that. I had my landline phone and a Google Voice account. I decided I didn't want the landline phone and its related expenses. So, I told Google to "port" my landline number to the Google Voice account. Now, when you call my landline number, it actually rings directly to my Google Voice account. Or, I can configure my Google Voice account to forward calls to my old landline number (now on Google Voice) to my cellphone.
What I was suggesting to you is:
1. Apply for a Google Voice account.
2. Select a phone number there.
3. When you get a new cellphone, configure Google Voice to forward all calls to that new number to your current cellphone.
4. Give out the Google Voice number as your "cellphone".
5. If you change cellphone numbers (actual not Google Voice), simply point your Google Voice number to the new cellphone.
The result is you never have to change your Google Voice number and it doesn't care which number you forward it to. You could even forward it to your house phone so people calling your "cellphone" (remember, you gave out your Google Voice number as your "cellphone") ring through to your house. No more waiting till you get into town where there is cell service.
Chris