Many people are using Google Voice for both GMail as well as Google Apps for Business accounts.  We've found a few cases where someone wants to transfer in, or transfer out a Google Voice assigned phone number between either Google Apps to Google Apps accounts, or from a GMail over to Google Apps account.

You can try to transfer by starting here: Google Voice Number Transfer*

* NOTE: This is not the same as "porting" your existing landline or cell phone number to Google Voice.

If the link above does not work, due to already having an existing Google Voice on both accounts, here is a quick tip to transfer with some help of a middle-man:

Here is the solution, if a user cannot transfer from two accounts, each with Google Voice... and you want to simply swap #'s...

Example:

• Account 1: 555-555-5555 (Google Voice Number)
• Account 2: 777-777-7777 (Google Voice Number)

1. Create a new account (temporary) on Gmail
   - Just make up anything as this will be a temporary holding account
2. Transfer # from Account 1 to Temp
   - Be patient!!! If you have a lot of data (calls, sms, etc) it can take a while - we've seen it take over an hour when it says "Transfer in Progress" 
3. Transfer # from Account 2 to Account 1
4. Transfer # from Temp to Account 2
5. Done

Risky, but it seems to work fine.