Which line is used is determined by your setting for Primary Line for the PHONE Port. If you have not changed the setting, it's the landline.
If you're not sure, you can call your cell phone and see what number is displayed, or call 1-800-437-7950 and you will hear your number spoken.
You can force a call to use whatever service you have for SP1 (usually Google Voice) by dialing **1 before the number.
You can force a call to use whatever service you have for SP2 (usually a SIP service) by dialing **2 before the number.
You can force a call to use the landline by dialing **8 before the number.