Everton:
I can't recommend a general setup, because providers are very different. Most of the quality-oriented ones, e.g. Callcentric and
VoIP.ms, proxy audio through their servers and generally handle NAT mapping and keep-alive on their end. Turn off STUN and any NAT related settings in the ATA, turn off (or somehow avoid) any SIP ALG in the router, and you should be fine. For providers where audio flows directly to/from their carrier, you need to make your system appear as if it's on a public IP. Unfortunately, many (most?) consumer routers have quirks that make it difficult, so you may need to experiment. Most providers offer configuration examples on their Web sites; some have a list of routers known to be incompatible.
My setup is unusual: We live in the US and have Phonepower as our primary service. However, we have a small apartment in Paris and stay here for six to eight weeks, twice a year. Phone service bundled with FTTH service from Orange includes "unlimited" free calling to landlines in 100+ countries, but has no BYOD access; I brought an OBi, primarily to be able to use the Orange service from elsewhere. We have prepaid cell service here, because all postpaid services for smartphones, i.e. that include a data plan, require a minimum 12 month contract; we'd be paying continuously for an expensive service used only 3-4 months per year.
Prepaid plans don't require a contract and have inexpensive data options. Although incoming calls are free, outbound domestic calls are a steep 0.50 €/min.; international is even higher. The usual solution (with smartphones) is to use the click-to-call service offered by many providers -- it rings both your cell and the destination, and bridges the calls. Some even have apps that integrate with the phone's contact list and do it all automatically. The cost with most VoIP providers to FR mobile is quite reasonable, typically ranging from $0.05 to $0.20/min. Unfortunately, a bureaucratic snafu caused a delay of several days getting the data plans activated, so I set up the OBi to do callback.
I chose Phonepower for the outbound leg (when calling the US), because those calls are free on our existing "unlimited" plan, and the caller ID sent is known by our contacts. PP is not suitable for the trigger call, because our IP phones here (not connected through OBi) would ring and disturb anyone at home. I happened to have a Paris DID with ippi (obtained because of a previous bureaucratic snafu, where the Orange home phone didn't work for ten days), so I used that for the trigger. For the inbound leg, I didn't want to use either ippi (expensive at 0.15 €/min.) or Phonepower (audio proxied through their California server would add four trips across the Pond to round-trip latency). I happened to have a Voxbeam account (another long story); their $0.0361/min. rate is excellent and the media server is only 14 ms from here. Bottom line: we can call the US from our cell phones for less than $0.04/min., get excellent quality, with only a little more hassle than click-to-call.