A page on the Asterisk Wiki entitled Calling using Google contains this bit of information about priorities:
More about Priorities
As many different connections to Google are possible simultaneously via different client mechanisms, it is important to understand the role of priorities in the routing of inbound calls. Proper usage of the priority setting can allow use of a Google account that is not otherwise entirely dedicated to voice services.
With priorities, the higher the setting value, the more any client using that value is preferred as a destination for inbound calls, in deference to any other client with a lower priority value. Known values of commonly used clients include the Gmail chat client, which maintains a priority of 20, and the Windows GTalk client, which uses a priority of 24. The maximum allowable value is 127. Thus, setting one’s priority option for the XMPP peer in res_xmpp.conf to a value higher than 24 will cause inbound calls to flow to Asterisk, even while one is logged into either Gmail or the Windows GTalk client.
Outbound calls are unaffected by the priority setting.
Therefore, if you are not receiving some incoming Google Voice calls, check to make sure that your priority= statement in the configuration is set to at least 25.
Read more: Calling using Google