The best way to set that up, IMHO, is to also integrate it with something like callcentric. Setup the basic "Incredible PBX" with built-in googlevoice free calling, then create a new trunk (trunk = outgoing phone line) for callcentric. Make it default to googlevoice, but if you dial 9 then a number, it goes through callcentric.
PAP2 adaptor is $30 (ebay, possibly fake) to $50 (Amazon). This is not necessarily a sunk cost if you abandon the plan since you can always sell it later for $30-$50 on ebay.
Callcentric cost without an inbound phone number ("DID") is zero if you dont use it, and $0.01+ per minute if you use it. International rates are very good but not the best around. If you want an inbound phone number from callcentric or any other provider, the cost is $3-$6 per month. Some include unlimited inbound with that fee.
Don't forget that with this setup, you do NOT have 911 service on the phone. This is generally $2 per month with any provider.