The OBi configuration generated by the OBiTALK Web Portal is very limited and only supports using the PrimaryLine when placing calls from OBiON Apps and other OBi's. Therefore, the only way to place a call through the PSTN line from an Android or iPad is to set your OBi's PrimaryLine to PSTN Line.
If you want complete flexibility to use any OBi trunk for making outbound calls, you will need to use a configuration such as the one described here :
Single-Stage Dialing Through Any OBi Trunk
