Assuming you have Google Voice working on SP1 and Callcentric working on SP2, set:
Service Providers -> ITSP Profile A -> General -> DigitMap:
(<1aaa>[2-9]xxxxxx|<1>[2-9]xxxxxxxxx|1xxxxxxxxxx|<**2>011xx.)
where aaa is your local area code.
Service Providers -> ITSP Profile B -> General -> DigitMap:
(*123|**275*xx.|<1aaa>[2-9]xxxxxx|<1>[2-9]xxxxxxxxx|1xxxxxxxxxx|011xx.|(Mipd)|[^*#]@@.'@'@@.)
where aaa is your local area code.
7/10/11 digits -> SP1 (Google Voice)
011+ -> SP2 (Callcentric)