The OBi has a setting that determines the PrimaryLine:
Physical Interfaces -> PHONE Port -> PrimaryLine
The PrimaryLine does not change unless you change this setting.
If the PrimaryLine is SP1 Service, then all outgoing calls will use SP1 unless you first enter **2, **8, or **9.
If the PrimaryLine is SP2 Service, then all outgoing calls will use SP2 unless you first enter **1, **8, or **9.
Etc.