Well, you can use the same
Voipo.com as your provider on say SP2. You could get their full service and get the SIP credentials from them and set that up on SP2 (assuming GV is set up on SP1). Their full service is more than $36 per year and normally, they like you to use their supplied ATA. However, if I'm not mistaken, they will supply the SIP credential for you to use on your OBi110. I think they say they will not provide any support for user supplied ATA. They run multi-year discount promotions ever so often, which works out to be of the order of around $130+ per year. Check with them or their Web Site for the pricing. Using
Voipo.com would avoid any porting fees in moving to another provider.
If I misunderstood your original question, please forgive. BTW, have you checked recently with GV to see if your number might be portable today? There are ways to check online to see if
Bandwidth.com (the supplier of GV numbers) has presence in your area.