When you list a phone, such as your cellphone, as a trust caller, when you call your GV number from your cell, and that comes into the OBi, the phone connected to the Obi will not ring immediately. If you then press 1, the attached phone will ring the same as if it were not a trusted caller.
If you instead press 2, OBi will ask for the PIN (personal ID number). After PIN entry, you can make phone calls out on the other line. These might be to places that would be expensive to call from your cellphone.
Pressing 3 gives the callback feature, which you might use, for example, if your cellphone has free incoming minutes.