i used **8 before dialing a number and the call quality was really good (other person can hear me well). Does the **8 mean I'm going through my landline number and not through my GV number?
Yes. Service route access codes can be used to override the PrimaryLine setting:
**1 + number -> SP1 Service
**2 + number -> SP2 Service
**8 + number -> PSTN Line
**9 + number -> OBiTALK Service