If you can connect the Manila OBi110 to a landline (and not have significant conflicts for use of it), that would be a good way for you to call other Manila numbers. Depending on the landline plan, it may also be a good deal for other PH locations and for mobiles -- you'll need to compare rates with the VoIP providers. Set up the OBi as detailed by RonR, except that you may want the AA's PrimaryLine to be PSTN Line.
For other international calls, IMO you should choose a provider that allows you to connect both by SIP (using a free or paid app on the iPhone) and as a "calling card". The latter method will use your cellular minutes, but will be useful when the data connection is not good enough for decent VoIP, which is usually the case in a moving vehicle.
The ratings below are just my personal opinion.
Top quality / reliability: Callcentric, Anveo.
Lowest cost: a Betamax brand, see http://backsla.sh/betamax
. Some brands offer a one-call free trial to inexpensive destinations.
Middle of the road: CallWithUs, Future-Nine, VoIP.ms.
In addition to the above conventional VoIP providers, you might also look at Localphone and Rebtel, who have a more convenient calling card service. Once set up, you dial a local number and it forwards automatically to your foreign contact. Both offer a five-minute free trial. Rebtel is higher quality (and more expensive). With Rebtel, your contact can optionally hang up and call the (usually local) number shown as caller ID. You are reconnected and the call is free to you (but your contact pays for a local call or cell minutes). This only works to some countries, including AU but not PH.
Of the companies mentioned, all provide SIP access, except Rebtel (you use their app). All have dedicated calling card access numbers, except Anveo and VoIP.ms; you must buy your own access number to use as a calling card.
Of course, you could use a second GV account to call overseas, though IMO it's not a good value for the countries you are calling. Over Wi-Fi or celluar data, use Talkatone for GV; over cellular voice, just dial your GV number and at the main menu, press 2 to make a call.