When you go thru the OBi server where you use the OBiON app to the server to your OBi adapter, the OBi server is in effect acting as a voip provider. OBihai makes it easy to call between the IPhone and an OBi adapter and you do not use up one of the two SPx voip provider configurations.
If you wish to use something other than the OBiON app, the easiest thing to do would be to get two accounts at a service provider like CallCentric's "IP Freedom" account where you can place calls between the accounts without charge. Setup one account on acrobits on the iphone, the other account on SP1 or SP2 on the OBi. Setup the incoming call routing (X_InboundCallRoute) for the CallCentric account on the OBi to go to the automated attendant (aa) which will give you the option (option 2) to place a new call. You could set it up so only calls from your CallCentric acrobits account will go to the aa. Since you have the pstn line setup as the primary line the call you then dial will go out on the PSTN line.
Acrobits has an example of how to setup the app for CallCentric. You could use any voip provider that allows free calling between accounts setup with the same provider. I mentioned CallCentric because you do not have to fund the account for the account type I mentioned.
