Assume you have four different ITSPs configured on the device; SP1, SP2, SP3 and SP4. Using the OBiTALK dashboard, you can click on the SPx that you want to use as the default, and check the box to use it as the default for outbound calls. This setting "sticks" until you change it.
Now, if you want to use one of the other ITSPs, on a per-call basis, simply prefix the dial string with **2, for example to use SP2, or **4 to use SP4, and so on. This only applies for the current call.
None of this has any effect on inbound calls.